ZOOLOGY AND BOTANY, MICROSCOPYj ETC. 141 



with glass needles on a spatula, the superfluous salt solution is 

 removed with blotting-paper, and the section is slowly immersed in 

 absolute alcohol and left there until all air-bubbles have been 

 removed. In all studies of bacteria by staining sections it is 

 important to remember that the staining is apt to fail in the case of 

 some of the micro-organisms, and thus give misleading negative 

 results ; the addition of some acetic acid or caustic potash before 

 staining will generally remedy this defect, although the sections are 

 somewhat impaired by these reagents. It is hardly necessary to 

 record Professor Weigert's warning that really good objectives are an 

 absolute necessity for this work. 



Preparing Fatty Acids.* — Mr. F. J, Allen boils up the fat or 

 oil with a not too strong solution of caustic soda or potash (liq. sod'dd 

 or liq. potassae) until the alkali is quite saturated and refuses to 

 absorb any more fat. When it has cooled filter it and add dilute 

 sulphuric or hydrochloric acid (stirring and warming at the same 

 time) until no more fatty acid separates. Boil for a second or two, 

 then set aside to cool. When cold, the fatty acid will be found in a 

 solid mass on the surface, and the liquid part may be thrown away. 



It is well to boil the acid in fresh water to purify it, when, on 

 cooling, it will be practically pure. 



To get crystals, it is simply necessary to melt a small quantity on 

 a slide, and spread it very thin ; it crystallizes on cooling, and must 

 be mounted " dry." 



Carmine Solution. t — Prof. H. Hoyer, believing in the great 

 superiority in all respects of carmine for animal tissues, strongly 

 recommends the following as avoiding the objections which exist to 

 the simple solution on account of the difficulty of keeping it, and 

 other disadvantages. He is able to speak from a year's trial. 



Dissolve 1 gr. of carmine in a mixture of 1-2 com. of strong 

 liquor ammonias and 6-8 c.cm. of water, and heat it in a glass 

 vessel in a sand bath until the excess of ammonia has evaporated. So 

 long as free ammonia is present large bubbles are formed in the fluid, 

 and the latter shows the usual dark purple-red colour of carminate of 

 ammonia. When the free ammonia has evaporated small bubbles 

 appear, and the solution takes a brighter red tint. It is now left to 

 cool and settle, and by filtering, the bright red deposit (to be used over 

 again) is separated from the neutral dark fluid, which by the addition 

 of chloral-hydrate can be kept for a long time. J 



If the carmine solution is mixed with 4-6 times its volume of 

 strong alcohol a scarlet-red precipitate is formed. This is separated 

 by filtration, washed and dried, or made into a paste with alcohol in 

 which some glycerine and chloral is dissolved. Both the powder and 

 the paste can be kept several months unchanged ; they dissolve easily 

 in distilled water, particularly the paste. The solution passes 

 readily through the filter, whilst the ordinary carmine solution can 



* Joum. Post. Micr. Soc, i. (1882) p. 193. 



t Biol. Centralbl., ii. (1882) pp. 17-19. 



X Cf. infra, p. 142, as to its iise in the preparation of a red injection-raass 



