HYDROCHT.ORTC ACID — HYDROGEN PEROXIDE 309 



plished by the withdrawal of water from them. This is in 

 most cases best accomplished by means of successively higher 

 grades of alcohol, as described on page 260. 



A quick method of hardening fresh tissues, and at the same 

 time preparing them for immediate sectioning, is to freeze them 

 by the evaporation of ether or the expansion of liquid carbonic- 

 acid gas. This process requires the use of special apparatus, 

 for a description of which the student is referred to the catalogue 

 of Bausch & Lomb, Rochester, N. Y. For an inibedding 

 mass, either a drop of the white of egg or a thick solution of 

 dextrin in a solution of carbolic acid i part, water 40 parts, 

 may be placed about the object before freezing. If the dex- 

 trine solution is used, it would be better to pump the air from 

 the object while immersed in the solution; then place the object 

 on the object-holder, pour a small amount of the solution about 

 it, and freeze. This method will answer very well in some cases 

 when it is desired to prepare a large number of sections quickly 

 for class use, but it can by no means take the place of fixing the 

 material in an appropriate fixative, hardening slowly in alcohol, 

 and imbedding in paraffin or collodion. 



The mucilaginous layer of certain seed coats may be har- 

 dened with a 10 per cent, solution of neutral acetate of lead. 

 The sections are cut from dry seeds, hardened in the lead acetate, 

 and stained with methyl blue. They are then washed in water 

 and mounted in a 2 per cent, solution of boracic acid. 



Hydrochloric Acid. — This reagent has such manifold appli- 

 cation in histology that its uses are best learned in the specific 

 cases of its application. See in the next chapter under Amy- 

 lose, Berberin, Caffeine, Calcium Oxalate, Calcium Sulphate, 

 Ethereal Oils, Magnesium Sulphate, Middle Lamella, Myro- 

 sine, Pectic Substances, Phloroglucin. Theobromine, Vanillin. 

 See also in this chapter under Maceration. 



Hydrogen Peroxide. — One part of hydrogen peroxide mixed 

 with 20 parts of 60 per cent, alcohol will, in a few minutes, remove 

 from sections the dark discoloration due to osmic acid which has 

 been used as a fixative (see page 263). 



