marsh gas, chlorine, bromine, phosphorus, phosphoric acid, and silicic acid. June 1867. = Hydrogen H, nitrogen = N, nitrous acid = N2O3, carbon = C, marsh gas = CH4, chlorine = Cl, bromine = Br, phosphorus P, phosphoric acid = H3PO4 (or P2O5), silicic acid = SiO2. 52. Give the atomic constitution and formulæ of alum, sulphate of magnesia, sulphate of soda, and calomel. July 1856. Alum, AIK(SO4)2 + 12 OH2, is constituted of 1 atom of aluminium, 1 of potassium, 2 of sulphur, and 8 of oxygen, crystallised with 12 molecules of water of crystallisation. Sulphate of magnesia, MgSO4 + 70H2, is constituted of 1 atom of magnesium, 1 of sulphur, and 4 of oxygen, crystallised with 7 molecules of water of crystallisation. Sulphate of soda, Na2SO4 + 100H2, is constituted of 2 atoms of sodium, 1 of sulphur, and 4 of oxygen, crystallised with 10 molecules of water of crystallisation. Calomel (mercurous chloride), Hg,Cl2, is constituted of 2 atoms of mercury and 2 atoms of chlorine. 53. Give the chemical names of the salts having the formulæ KCl and Na2O.CO2, and detail the means by which they can be discriminated. July 1859. = = KCI potassic chloride, and Na2O.CO2 (or Na2CO3) sodic carbonate or disodic carbonate. If any acid, say hydric sulphate, be poured into a solution of a carbonate, an effervescence takes place, and the carbonic dioxide is expelled, causing lime-water to become turbid. This does not take place with potassic chloride when so treated, On the other hand, add to the solution of potassic chloride a few drops of hydric nitrate and argentic nitrate. A white precipitate of argentic chloride is formed, which result is not seen if sodic carbonate be treated in like manner. 54. Give the formulæ of the following substances: Water, ammonia, silica, carbonic acid, sulphuric acid, marsh gas, and phosphuretted hydrogen. Jan. 1869. Water = OH2, ammonia = NH3, silica = SiO 2, carbonic acid = CO2, sulphuric acid = H2SO4, marsh gas = CH1, phosphuretted hydrogen = PH3. 55. Give the formulæ of potassic nitrate and potassic nitrite. June 1880. Potassic nitrate has for formula KNO3, potassic nitrite KNO2. 56. What is meant by the term crystal? Describe some of the processes by which crystals may be formed. July 1860. A crystal is the regular geometrical form which certain solid bodies assume when placed in favourable circumstances, the form being bounded by planes, which are inclined at angles to one another under certain laws. As an example of crystallisation melt sulphur, and after it has cooled and formed a solid crust at its surface, break the crust and pour off the still liquid portion of the sulphur, when a beautiful crystallisation will be found to have taken place. Crystals may be formed by the process of fusion, which consists in melting the substances so as to give to the particles freedom of motion among each other, as mentioned above in the case of sulphur; by sublimation, which consists in heating a body till its particles rise in vapour, as is the case with sulphur raised to a temperature of 490° C.; and, lastly, by solution, which consists in dissolving the substance and evaporating it either by heat or by exposure to the air, as is the case with sulphur dissolved in carbonic disulphide.1 57. Explain what is meant by dimorphism, and give examples of dimorphous bodies. Jan. 1860. A body that crystallises into two systems is said to be dimorphous, from two Greek words signifying twoshaped. Sulphur iş dimorphous, because when melted it crystallises in a shape belonging to the oblique prismatic system; when dissolved it crystallises in a shape belonging to the right prismatic system. Carbon also is dimorphous. 58. What are the acid solvents of mercury, silver, gold, and platinum? July 1853. The solvent of mercury (Hg) and silver (Ag) is hydric nitrate. The solvent of gold (Au) and platinum (Pt) is aqua regia, which is a mixture of hydric nitrate and hydric chloride. 1 Note that the crystals are not formed during fusion, evaporation, or solution, but during solidification, condensation, and deposition. |