92 THE MICROSCOPE. 



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 then removed by means of a hair pencil, 

 and a drop of the diluted sulphuric acid added j 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 the 

 case of objects, the consistence of which differs in different parts, the 

 section should be carried 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. 



The preceding observations must be received with caution ; a very 

 useful warning having been recently given by Dr. Parkes in reference 

 to unusual crystalline forms he has found some of these re-agents to 

 assume, when in use, under the microscope. Without a knowledge of 

 this singular fact, and a perfect recognition of the crystalline forms, 

 errors in micro-chemical research cannot but occur. For example, if 

 a drop of liquor potassa3 be allowed to evaporate on a slip of glass, 

 crystals appear, chiefly 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. 

 These crystals do not arise from the presence of impurities ; at least 

 perfectly pure potash exhibits the same phenomenon. 



He also states that the form of the crystals of acetate of potash 

 varies according to the strength of the acid out of which it crystallises ; 

 and when formed out of strong acid, very much resembles that of the 

 crystals of uric acid ; for, mixed up with other forms, long dagger-like 

 or lancet-shaped crystals are seen, which might well deceive us. 



It may also be noticed in this place, that Majendie has lately pointed 

 out that in certain albuminous mixtures iodine loses the property 

 of colouring starch blue. This difficulty must be got rid of before 

 iodine can be used without fear in micro-chemistry. 



Preparation and Preservation of Algce, &c. Mr. Half's excellent 

 directions for making preparations of the alga3 for the microscope are 

 as follows : " First to obtain a fluid which shall preserve the plant as 

 little altered as possible from its appearance when living ; and secondly, 

 to adopt the best means for preventing the escape of this fluid after 

 the object has been mounted in it. With respect to the first point, 

 the fluid which I have found to answer best is made in the following 



