WITH THE MICROSCOPE. 203 



be purchased for about is. 6if. a pound, but only very small quan- 

 tities are required. 



294. Hydrochloric Acia may be obtained perfectly colourless. 

 It should be kept in the pure state and diluted as required. 



295. Acetic Acid. Two specimens of acetic acid will be found 

 convenient. One, a solution of the strongest acid which can be 

 procured ; the other containing about twenty per cent. The last is 

 prepared by dissolving one part of the strongest liquid acid, or of the 

 pure glacial acetic acid, in five of water. 



The glacial acetic acid is now commonly employed for photo- 

 graphic purposes, and can, therefore, be very readily obtained. It 

 possesses great advantages over other kinds of acid for microscopical 

 purposes. 



296. Chromic Acid is usually required very dilute. For the 

 purposes of hardening tissues a watery solution of a straw colour will 

 be found strong enough. It is easily prepared by dissolving a little 

 of the crystallised chromic acid in distilled water. 



The crystallised acid may be prepared by decomposing roo 

 measures of a saturated solution of bichromate of potassa, by the 

 addition of 120 to 150 measures of pure concentrated sulphuric acid. 

 As the mixture becomes cool, crystals of chromic acid are deposited, 

 which should be dried and well pressed on a porous tile, by which 

 means the greater part of the sulphuric acid is removed, and the 

 crystals obtained nearly pure. 



297. Effects of Acids 011 Organic Structures. The effects of the 

 application of cold strong acids to animal textures are very variable ; 

 in some instances the tissue is completely destroyed, while in others 

 scarcely any effect seems to be produced. The mineral acids gene- 

 rally coagulate albuminous tissues, and render their microscopical 

 characters confused and indistinct. Tribasic phosphoric acid, how- 

 ever, is an exception to this. 



Acetic acid dissolves many of the substances allied to albumen. 

 The appearance of some textures is scarcely altered by the applica- 

 tion of a strong acid ; for instance, the blood corpuscles shrink a 

 little, but exhibit their usual form and general characters for some 

 time after the addition of strong nitric acid, and the cells of the 

 epidermis and nail, although turned of a yellow colour, are not 

 destroyed ; the latter are separated somewhat from each other, and 

 their outline is often made beautifully distinct. Most of the mineral 

 constituents of the body, insoluble in water, are directly dissolved by 

 the acids. Strong nitric acid is a very useful reagent for demon- 

 strating vegetable cellular structures. 



Acetic Acid. Acetic acid is one of the most useful reagents to 



