128 Testing Milk and Its Products. 



zontal position. In this way fresli solutions ready for 

 testing are always at hand. The cylinder is kept tightly 

 corked while the tablets are dissolving, so that none of 

 the liquid is lost by the shaking. It is well to put the 

 tablets in the cylinder with water at night ; the solution 

 will then be ready for use in the morning. Excepting 

 a flocculent residue of inert matter, "settlings," which 

 will not dissolve, the tablets must all disappear in the 

 solution before this is used. The strength of the tablet 

 solution does not change perceptibly by standing, ai 

 least for one week. The only precaution necessary is 

 to avoid evaporation of the solution by keeping the cyl- 

 inders tightly corked. The solid tablets will not change 

 if kept dry, any more than dry salt changes by age. 



141. Accuracy of the tablets. The tablets have been 

 repeatedly tested by chemists and found to be accurate 

 and uniform in composition. Tests made Avith the 

 tablets according to the directions here given can there- 

 fore be relied on as correct. The alkali solution is very 

 sensitive, however, and should not be measured in a cyl- 

 inder which has been previously used for measuring 

 sulfuric acid, as the smallest drop or film of acid from 

 a dish or from the operator's fingers will change the 

 strength of the standard tablet solution. 



141a. Powdered sodium carbonate weighed out in 

 the exact quantity required for making a gallon of 

 tenth normal solution has of late been placed on the 

 market; these "test powders" are cheaper than alka- 

 line tablets and when put out by a reliable firm are 

 equally as accurate as these. 



