MICROSCOPICAL PREPARATIONS. 9 



portion of the cell contents is thereby coloured bright yellow, 

 whilst the glycogen assumes a reddish-brown colour. A 

 further proof that the coloured part of the cell contents really 

 consists of glycogen is obtained by warming the preparation 

 to 70 C., whereby the colour reaction of the glycogen dis- 

 appears ; on cooling down the colour reappears. If the cells 

 are cautiously pressed under the cover-glass, so that they 

 burst, it may be observed that the brown coloured glycogen is 

 liberated as a fluid mass, which quickly dissolves in the sur- 

 rounding liquid. 



Oil or fat globules are often found amongst the granules 

 that occur in the fully developed yeast cells. By the addition 

 of 1 per cent, osmic acid they are coloured brown ; sometimes 

 they assume a vermilion colour with alcanna tincture, and 

 frequently they may be dissolved by treatment with alcohol- 

 ether, benzene, or carbon bisulphide. The treatment must, 

 however, be continued for some time ; the solution takes 

 place more rapidly if the cells are crushed and the solvent 

 brought into direct contact with the globules of oil. The 

 albuminoids of the cells, as already stated, are coloured yellow, 

 or yellowish-brown, with iodine ; with nitric acid and ammonia 

 they are coloured lemon-yellow, and with Millon's reagent 

 (mercuric nitrate), brick-red. 



In the examination of fermented liquids micro-chemical 

 methods are used to determine the character of any suspended 

 matter other than micro-organisms. To clearly recognise the 

 reaction of the tiny particles floating in the liquid, it is some- 

 times necessary to separate them by centrifugal action, and 

 further to wash them free from the liquid. The starch or 

 paste cloudiness which occurs in beer is produced by fine 

 particles of starch or dextrin, which separate out when the 

 liquor contains a given quantity of alcohol. They may be 

 recognised by the addition of iodine, whereby they are coloured 

 blue or brownish-red (dextrin). A cloudiness due to albumi- 

 noids is often produced ; it is due to flocculent, membranous, 

 and often very irregular agglomerates, and to minute granules, 

 which are either isolated or enclosed in these secretions. Such 

 a formation frequently occurs in pasteurised beer ; it may 

 be distinguished under the microscope by the readiness with 



