of A communicates with an apparatus for supplying a regulated stream of pure dry oxygen. The oxygen is stored in a gas-holder, from which it is passed through two bottles of potash solution to absorb carbon dioxide, and then through two bottles of concentrated sulphuric acid and a large U tube containing calcium chloride to absorb moisture. The bulbs B are closed by caoutchouc caps and weighed; the ends of C are also closed and the tube is weighed. A small porcelain boat (indicated by a in the fig.) is strongly heated, allowed to cool, and weighed; a little bit of diamond is then placed in the boat which is again weighed. The boat is now placed in the tube A and the apparatus is arranged as shewn in the figure. The copper oxide, b, is gradually raised to full redness, a slow stream of oxygen is passed through the apparatus, and the diamond is heated until it burns in the oxygen. The product of the burning is carbon dioxide, which is absorbed by the potash in B and C. When the burning is finished the apparatus is allowed to cool; B and C are weighed together, with the caoutchouc caps on; and the boat is withdrawn and weighed. A very little ash remains when diamond is burnt; the weight of ash found is deducted from the weight of diamond used, and the difference gives the weight of pure carbon burnt to carbon dioxide. The increase in the weight of B and C gives the weight of carbon dioxide formed. Assuming that the sole product of the burning was carbon dioxide, the results obtained enable the composition of this compound to be established.

The results of this experiment, if very carefully conducted, shew that 8 parts by weight of oxygen combine with 3 parts by weight of carbon to form 11 parts by weight of carbon dioxide. The molecular weight of carbon dioxide is therefore n11, where n is a whole number. We must now determine the value of n. An approximately accurate determination of the specific gravity of the gas carbon dioxide will evidently suffice for this purpose.

Clean and dry a light flask of about 250 c.c. capacity. Close the flask by a very good well-fitting cork fitted with a short exit tube closed by a piece of caoutchouc tubing and a screw-clamp, and weigh it. In weighing the flask it is advisable to place another similar flask in the other pan and then to add weights until equilibrium is established. Note the temperature and pressure of the air. Pass a fairly rapid

stream of pure, perfectly dry, carbon dioxide into the flask by a tube reaching to the bottom; after 10 or 15 minutes very slowly withdraw this tube and instantly cork the flask, taking care that the cork is pushed into the neck to the same distance as when the first weighing was executed. Open the screw-clamp for a moment to establish equilibrium of pressure within and without the flask, and again at once close the clamp. Weigh again. Open the flask under potash solution (1 of potash in 2 of water); if the potash does not rush in and entirely fill the flask (shewing that the flask contained only carbon dioxide) begin the Exp. again. Make a mark on the neck of the flask at the point to which the cork reaches. Remove the cork; fill the flask to the mark on the neck with water and determine the volume of this water; let it be a c.c. You have thus determined the capacity of the flask. Calculate the volume which the a c.c. of air at the observed temperature and pressure will occupy at 0° and 760 mm.; then, knowing that 1000 c.c. of air at 0° and 760 mm. weigh 1.293 grams, find the weight in grams of this volume of air; that is, find the weight of air in the flask when it was weighed full of air. Now deduct this weight from the observed weight of flask and air; the difference is the weight of the empty flask. Deduct the weight of the empty flask from that of the flask filled with carbon dioxide; the difference is the weight of the carbon dioxide.

You now know the weights of equal volumes (a c.c.) of air and carbon dioxide at the same temperature and pressure. From these data find the specific gravity of carbon dioxide referred to air; multiply this number by 28.87, and you obtain the specific gravity of the gas referred to hydrogen as twice unity, that is you obtain an approximately accurate value for the molecular weight of carbon dioxide.

The result of this Exp. shews that the molecular weight of carbon dioxide is about 44. But you have already found that 3 parts by weight of carbon combine with 8 parts by weight of oxygen, and that the molecular weight of carbon dioxide is therefore n11; you have now found the value of n to be 4.

As 8 parts by weight of oxygen combine with 3 parts of carbon, there must be 32 parts by weight of oxygen combined with 12 parts of carbon in 44 parts by weight, that is in a molecule, of carbon dioxide.

The results of Exp. 2 shewed that the atomic weight of oxygen is not greater, although it may be less, than 16; the

M. P. C.

9

results of the present Exp. shew that the atomic weight of oxygen is probably not less than 16.

The results of the present Exp. also shew that as 12 parts by weight of carbon combine with oxygen to form a molecule of carbon dioxide, the atomic weight of carbon is not greater than 12.

Exp. 4. Determine the composition and molecular weight of carbon monoxide; assuming the specific gravity of the gas to be known.

Carbon monoxide gas is about 14 times heavier than hydrogen; therefore its molecular weight is approximately 28.

Fill a eudiometer with clean and dry mercury and invert it in a mercury-trough. Arrange a small flask with a funnel tube, and an exit tube connected with a bottle containing potash solution to absorb any carbon dioxide produced in the reaction; let the exit tube from this bottle be somewhat narrowed at the open end and bent so that this end can be easily brought under the open end of the eudiometer in the trough. Place about 20 c.c. of concentrated formic acid solution and about 100 c.c. of concentrated sulphuric acid in the flask, and gently heat until a stream of carbon monoxide passes through the drying bottles (H,CO2 – H2O – CO).

As carbon monoxide is very poisonous these experiments must be conducted in a draught place.

After the lapse of 10 minutes or so you may be sure that the issuing gas is free from air. Now allow about 30 c.c. of the pure carbon monoxide to pass into the eudiometer; withdraw the lamp; and set aside the eudiometer. Meanwhile put about 5 grams of pure dry potassium chlorate in a tube of hard glass; draw out the open end of the tube and bend it into the shape of the delivery tube of an apparatus for making and collecting oxygen or hydrogen. Place this tube in a clamp, and heat it gradually until a rapid stream of oxygen is evolved. While this is proceeding, read off the level of the mercury in the eudiometer, also read the thermometer and barometer, and the height of the column of mercury in the eudiometer, and so determine the volume of carbon monoxide. By this time pure oxygen, free from air, will be issuing from the tube in which the potassium chlorate is heated; cause the oxygen to be evolved rapidly, then remove the lamp, and quickly carry the tube to the eudiometer into which pass about 80 to 100 c.c. of oxygen. Press down the eudiometer firmly on a pad of india-rubber moistened with a solution of

mercuric chloride, and clamp it; then pass an electric spark through the gases; the carbon monoxide is burnt to dioxide (CO+O = CO2). Now slowly remove the eudiometer from the pad and allow the mercury to rise. Read the level of the mercury in the eudiometer, also the height of the column of mercury in the eudiometer; these give the volume of carbon dioxide plus oxygen now in the eudiometer.

Bring a few c.c. of a solution of potash (1 part potash in 2 of water) into the eudiometer by means of a bent pipette. The carbon dioxide is absorbed; when absorption is complete, read the level of the liquid in the eudiometer, and the height of the column of mercury; the contraction which has occurred, making allowance for the short column of potash solution the specific gravity of which may be taken as 1.32, measures the carbon dioxide in the tube when the potash was added.

The

You have now determined that a certain volume of wet carbon monoxide, say a c.c., at a specified temperature and pressure, when burnt gave a certain volume, say b C.C., of wet carbon dioxide at a specified temperature and pressure. volumes of the dry gases at 0° and 760 mm. must now be calculated; then, knowing that 1000 c.c. of carbon monoxide at 0° and 760 mm. weigh 1.251 grams, and 1000 c.c. of carbon dioxide at 0° and 760 mm. weigh 1.966 grams, it is easy to find the weights of the volumes of the two oxides.

When this calculation is made, you have determined that a certain weight, a grams, of carbon monoxide when burnt gave a certain weight, b grams, of carbon dioxide; as you already know the gravimetric composition of carbon dioxide it is easy to find the weight of carbon in these b grams; but this is also the weight of carbon in the a grams of carbon monoxide; hence, as this oxide is a compound of carbon and oxygen only, you have the data for finding the composition of carbon monoxide.

The result of this experiment, if carefully conducted, is that 8 parts by weight of oxygen are combined with 6 parts of carbon in 14 parts of carbon monoxide.

The molecular weight of the gas is therefore n14 where n is a whole number; but n must be equal to 2, because carbon monoxide is about 14 times heavier than hydrogen.

A molecule of carbon monoxide is therefore composed of 16 parts by weight of oxygen combined with 12 parts by weight of carbon.

The results of this experiment confirm the numbers 16 and 12 as the atomic weights of oxygen and carbon respectively.

CHAPTER IV.

DISSOCIATION.

Exp. 1. DETERMINE the specific gravity (referred to hydrogen as unity) of the vapour obtained by heating ammonium chloride. For this purpose use v. Meyer's apparatus heated in a bath of molten lead to a temperature of about 500° or so. The bath may consist of a piece of iron gas-pipe about 21 inches wide and 8 or 10 inches long closed at one end by an iron plug screwed on. The lower end of the vapour density apparatus is rested on the surface of the solid lead; the lead is gradually melted by means of a large Bunsen-lamp (lead melts at about 365o), and the apparatus is allowed slowly to sink into the molten metal.

Assuming the atomic weights of nitrogen and chlorine to be 14 and 35.5 respectively, the simplest formula that can be given to ammonium chloride consistently with its composition is NH Cl. If this formula represents the composition of a molecule of the compound in the state of gas, the molecular weight of gaseous ammonium chloride must be 53.5, and the gas must be about 26 times heavier than hydrogen

But the result of your experiment shews that the vapour obtained by heating ammonium chloride is about 13 times heavier than hydrogen. You would therefore be inclined to deduce the number 26 as the approximate value of the molecular weight of ammonium chloride gas; but if this value is correct, either the formula NHCl must be given to the compound, or the accepted atomic weights of nitrogen and