cold water, so that the fat is very near the bulb of the instrument. This beaker is placed inside another larger one also containing water, and thus a water bath is improvised-by means of which the temperature of the water in the smaller beaker can be slowly and uniformly raised by the application of heat to the larger beaker. The temperature recorded by the thermometer at the exact stage at which the fat melts, is that of the "melting point."

The melting points of the different fats vary within narrow limits, owing to the variable quantities of stearin, olein, and volatile fatty acids; and this test, therefore, can never prove complete purity, since small amounts of some of the other fats-and even large amount of beef-dripping—would not be detected.

The salt in butter may be estimated thus:

Weigh out 5 grammes of butter into a weighed platinum dish, and place for about two hours over the water bath to drive off water-stirring frequently with a glass rod or the water will not be able to escape. Then extract fat with warm ether, in the manner described in "milk analysis." Dry the residue, dissolve out the salt with warm distilled water, and estimate the amount of sodium chloride from a quantitative estimation of chlorine-performed as in water analysis.

An instrument recently invented by M. Soudén, and termed the "liquoscope," will probably come to be employed as a rapid means of detecting adulteration in butter, oils, glycerine, &c. It consists of a glass cylinder containing glycerine, into which two hollow prisms are immersed. When substances with identical refractive indices are placed in the prisms, and a black horizontal line upon a white background is looked at

through these, the line appears continuous; but when the substances are of different refractive indices, the line, when thus viewed, appears broken in two-one part being at a higher level than the other.

MICROSCOPIC EXAMINATION.

Pure butter, under the microscope, is seen to consist of a collection of small round highly refractile oil

FIG. 49.-Butter-fat. (× 350).

globules, together with a few much larger ones of fairly uniform size. The larger globules average about 6 in number in the field of the microscope at the same time, while the smaller would number many thousands.

In oleo-margarine the small oil globules are much less distinct in contour, i.e., they stand out less markedly

FIG. 50.-Oleo-margarine. (× 350).

than in butter fat and have the appearance of being more crowded together; on the other hand, there is, relatively, a much greater number of the larger globules, some of which reach a much greater size than others, and these do not, in consequence, present the same uniformity as was noted in the case of pure butter.

ADULTERATION.

Lard, beef mutton and horse fats, together with vegetable oils (linseed, palm and cocoanut), are employed as substitutes of butter fat.

Water is rarely worked into a butter in excessive quantities for fraudulent purposes, since water exceeding about 16 per cent., is decidedly prejudicial to the keeping powers of the butter. Such a practice, however, increases the weight, and is often resorted to in those cases where a ready market is ensured, and the keeping powers of the butter are not, therefore, taxed before sale. Common salt is added to improve the flavour, and also to preserve the butter by checking the decomposition of the casein. It has been found to reach as high as 14 or 15 per cent.; but seeing that its presence in such excess gives a markedly saline taste, it becomes a question of individual palate as to whether such butter is bought and consumed; the point is not one of public health import, and rarely does the percentage amount exceed that which will lend a palatable amount of saltness to the butter, i.e., about 5 or 6 per cent.

The addition of potato or other starch would be best detected by the microscope, after treating with iodine; but these are practically never employed for adulteration purposes.

Annatto, turmeric, and other harmless colouring agents, are commonly used, and C. F. Cassal first pointed out that boric acid was frequently to be found in butter and cream, and that it plays an important part in the composition of the numerous proprietary nostrums sold for preserving these articles.

CHEESE.

Cheese consists of the original constituents (chiefly the casein and fat) of the milk from which it is made; but, as the process of ripening proceeds, the sugar becomes changed-chiefly into lactic acid.

The average composition of Stilton,* Gorgonzola, and Dutch cheese-as shown by Bell's analysis, is :

There is remarkably little adulteration practised in the manufacture of cheese, and the cause is not far to seek, for the article does not readily lend itself to sophistication. For the curd, which is separated from milk by rennet, there is no spurious and cheap substitute which can be made to yield the peculiar characters of pure cheese; there seems, however, a strong probability that lard and animal and vegetable fats may eventually become extensively employed in the manufacture of the cheaper cheeses-so that, by making these replace the abstracted fat (cream) of skimmed milk, this latter may be turned to account.

Different samples of each of these cheeses vary considerably in the percentage amounts of their constituents.

+ Very poor because made from milk which has been partially skimmed.

In this country, at the present day, it may be said that with the exception of colouring matter-which is generally annatto and harmless-the cheeses produced are pure. It has been shown, however, that in some cases (especially foreign cheeses) their surfaces have been brushed over with highly poisonous antiseptic solutions, such as arsenious acid and sulphate of copper-in order to preserve them from attack by parasites and from decay; that colouring matters have also been used to tint the rind; and that those small and delicate cheeses which are wrapped in thin lead papers may take up the metal. A careful examination, therefore, of the rind for metallic poisons may be occasionally desirable.

000

FIG. 51.-Aspergillus glaucus.

Any starch employed-as it rarely is-as an adulterant, can be detected by the iodine reaction and the microscope.

Cheese is peculiarly liable-and especially the moister kinds to parasitic growths, which are considered by many to add materially to the flavour of the article. They are probably harmless when ingested with the cheese; but a knowledge of the commoner forms of growth is desirable :

Aspergillus glaucus is a form of vegetable fungus, and gives rise to the appearance popularly known as "blue