PRACTICAL MICROSCOPY. 



Fig. 177 is an illustration of a section of a lichen thallus, 

 and Fig. 178 a similar slice from the apothecium: mounted 

 sections should show the details as well as these. 



FIG. 177. 



FIG. 178. 



Animal tissues require different treatment. They are 

 generally too soft and pulpy for immediate cutting, so 

 require to be soaked for various periods in some hardening 

 solution ; while others, such as porcupine-quill, whalebone, 

 and horn, require immersing in hot water to soften them. 



For hardening animal tissues, alcohol, and chromic acid 

 solution are most generally used, while some prefer bi- 

 chromate of potash, or bichloride of mercury. Alcohol is 

 perhaps the safest of all hardening agents ; it being a 

 powerful abstractor of water, coagulating albumen, and 

 acting also as a preservative agent. 



Chromic acid is to be obtained in beautiful dry carmine 

 crystals. It is more conveniently kept as a stock solution, 

 I ounce being dissolved in 50 ounces of water, and diluted 

 as occasion requires. 



It must not be forgotten that tissues are easily over- 

 hardened and rendered brittle by prolonged immersion in 

 chromic acid, and therefore the weaker a solution can be 

 used the better. Some tissues require hardening by special 



