394 GENERAL MICROSCOPIC TECHNIQUE. 



of the microtome. Whereas in cutting celloidin sections nearly 

 the whole edge of the knife is used, in paraffin sections only a 

 small part of the edge is used. With large objects, however, 

 the knife is placed obliquely in cutting paraffin sections also. 

 The temperature of the room has an important influence on the 

 cutting of paraffin sections. If the paraffin is of the right con- 

 sistency, the sections should be quite flat, and should adhere to 

 one another at the edge, so that what is known as a " ribbon " 

 of sections is obtained i. e., they are fastened together in per- 

 fect sequence. Such ribbons may sometimes be obtained a foot 

 or more in length, consisting of what are called serial sections. 



If the paraffin sections do not flatten as they should, they 

 may be made to do so by floating them on warm 30 per cent." 

 alcohol, or on warm water. This should not be hot enough 

 to melt the paraffin. From this they may be lifted out on a 

 glass slide and allowed to dry in a thermostat (at 35 C.) for from 

 twelve to forty-eight hours. This treatment causes the sections 

 to adhere firmly to the slide, so that they may be stained and 

 mounted. In order to be successful in this method, the slides 

 which are used should be entirely free from fat or dirt of any 

 sort. Simple washing with soap or alcohol is not sufficient. 

 They should be washed with soap and water, boiled in a solu- 

 tion of potassium hydroxide, and allowed to remain several 

 days in concentrated sulphuric acid. They are then washed in 

 distilled water and kept until used in a mixture of alcohol and 

 ether. They should be handled only with clean instruments, 

 and not with the fingers. 



Instead of this method, many so-called fixatives may be 

 used. These are by no means so good. A common method is 

 to coat the slide with a thin layer of albumin. The sections are 

 pressed down on this and the albumin coagulated by heat. 

 This solution of albumin is made as follows: egg albumin, 

 glycerin, aa 50 grammes ; sodium salicylate, 1 gramme. 



Paraffin sections, after they have been fastened firmly to the 

 slide by one of these methods, should be placed in xylol for 

 five minutes. This dissolves the paraffin out of the tissue. 

 The xylol should be changed once. If the tissue has already 



