346 REAGENTS AND THEIR PREPARATION [Cn. X 



REAGENTS FOR MICROSCOPIC WORK 



533. For much of the work done with a microscope the re- 

 agents needed are few and inexpensive, but for a large laboratory 

 with the diversity of investigations carried on the reagents are nu- 

 merous, and some of them expensive. Below are given some of the 

 principal ones with the method of their preparation. 



534. General on preparation of reagents. In preparing re- 

 agents both weights and measures are used. As a rule the amounts 

 given are those which experience has shown to give good results. 

 Variations in the proportions of the mixtures are sometimes advan- 

 tageous, and in almost every case a slight change in the proportions 

 makes no difference. Most laboratory reagents are like food, good 

 even under quite diverse proportions and methods of preparation. 

 With a few, however, it is necessary to have definite strengths. 



By a saturated solution is meant one in which the liquid has dissolved 

 all that it can of the substance added. This varies with the tempera- 

 ture. It is well to have an excess of the substance present; then the 

 liquid will be saturated at all temperatures usually found in the 

 laboratory. 



535. Solutions less than 10 per cent. In making solutions where 

 dry substance is added to a liquid, if the percentage is not over 10 %, 

 the custom is to take 100 cc. of the liquid and add to it the number of 

 grams indicated by the per cent. That is, for a 5 % solution one would 

 take 100 cc. of the liquid and 5 grams of the dry substance. This 

 does not make a strictly 5 % solution. For that one should take 95 cc. 

 of liquid and 5 grams of the dry substance; or, If the percentage must 

 be exact, then one should weigh out 95 grams of the liquid and add 

 5 grams of the dry substance. 



536. Solutions of 10 per cent and more. When the percentage 

 is 10% or over it is better to weigh out the number of grams repre- 

 senting the percentage and add to it the right amount of liquid in 

 cubic centimeters. For example, if one were to make a 35% aqueous 

 solution of caustic potash in water then one would add 35 grams of 

 caustic potash to 65 cc. of water. If one wished to make a 10% 

 alcoholic solution of caustic potash he would add 10 grams of caustic 



