326 HOW TO SEE WITH THE MICROSCOPE. 



among the previous drops on the paper and see if there 

 are any indications of the yellow. If so, you have used 

 too much haste, and the analysis must be repeated. If to 

 the contrary the yellow is only to be observed at the last 

 test made on the paper, the analysis is ended. Read the 

 burette, multiply this number by 12 -r- 100 and you will 

 have the number of grains to the fluidounce of urine. 



It will be well, before making a practical use of the 

 standard solution, to test the same. Proceed thus : 



Procure, say one-fourth ounce of pure urea. This can 

 be had of the leading druggists at a cost of about twenty- 

 five cents. Of this weigh carefully two grains, which 

 dissolve in 200 grains of water. Take 50 grains of the 

 solution and tritate as above. When the carb. of soda 

 paper strikes a yellow color, desist and read the burette. 

 Now the 50 grains of solution contained, of course, one- 

 half grain of urea, and the burette should, if the stand- 

 ard solution have been properly prepared, read at the 100 

 mark. If the reading should be less than the 100, the 

 solution is too strong, and must be weakened by the fur- 

 ther addition of distilled water. If, on the contrary, 

 the reading should be more than 100, the solution is too 

 weak, and more of the mercury must be prepared and 

 added. For the purposes of the practitioner there is 

 no necessity of being over precise. If the burette shows 

 but a small error, plus or minus, it will be sufficient to 

 note the fact on the label of the bottle, applying a 

 proper correction to the future analyses. In the exam- 

 ination of the reactions on the carb. of soda paper, es- 

 pecially in the evening, we find a hand-magnifier of 

 much service. 



