CHEMICAL RE-AGENTS. 247 



minute, or a minute, in a watch-glass : boiling is here un- 

 necessnry. The section is taken out, and treated with 

 water in another watch-glass. 



9. Oil of Lemons, or any other essential oil, a drop of 

 which will be found of value in the investigation of pollen 

 and spores. 



Lastly may be enumerated a pretty strong solution of 

 Carbonate of Soda and also of Acetic Acid; which latter, 

 however, is more especially useful in the investigation of 

 animal tissues. 



To the above may be added a test for protein compounds. 

 This test is composed of sugar and sulphuric acid, and is 

 thus employed: — A thin section or portion of the tissue to 

 be examined is placed in a drop of simple syrup, this is 

 ^hen removed by means of a hair pencil, and a drop of the 

 diluted sulphuric acid added ; the red colour usually does 

 not appear until after the lapse of about ten minutes. 



In making thin sections of tissues, it is recommended 

 that, in those o1)jects the consistence of which differs in 

 different parts, the section should be carj'ied from the 

 harder into the softer portion; also, in making a thin 

 section of a very minute yielding substance, to enclose it 

 between two pieces of cork, and to slice the whole together. 

 It is also useful sometimes to saturate the object with 

 mucilage, which is to be allowed to dry slowly; in this 

 way very delicate tissues may be sliced, or otherwise 

 divided without injury, and with great facility. 



Some of the above re-agents must be used with caution, as 

 it is not unusual for them to assume crystalline forms while 

 under the microscope. Without a knowledge of this fact, and 

 a perfect recognition of crystalline forms, errors in micro- 

 chemical research must occur. For example, if a drop of 

 liquor potassse be allowed to evaporate on a slip of glass, 

 crystals appear, chiefly of six-sided tables, precisely like 

 cystine; when in quantity, they are often crowded together 

 as the cystine plates are, and sometimes exhibit a similar 

 nucleus-like body in their centres. This peculiarity of 

 crystallization does not arise from the presence of impurities; 

 perfectly pure potash often exhibits the same phenomenon. 



The form of the crystals of acetate of potash varies 

 according to the strength of the acid out of which it, 



