PART I. 

 PUBLIC HEALTH CHEMISTRY. 



IT is necessary, in the first place, to consider carefully 

 certain points of chemical practice, which form the 

 basis of much of the subsequent work, and which must 

 be thoroughly understood to enable the latter to be 

 readily followed and apprehended. 



CHAPTER I. 

 CHEMICAL ANALYSIS. 



THIS is of two kinds Qualitative or Quantitative. 



i- Qualitative. Consists in proving the presence or 

 absence of certain metals or salts, or generally of chemical 

 elements or radicles or compounds in a substance or 

 solution, by the use of a series of tests. 



2. Quantitative.- Consists in separating out the con- 

 stituents of any composite body and accurately estimating 

 the amount of each of them. This may be done in three 

 ways : Gravi metrically, Volumetrically, Colorimetrically. 



Gravimetric method. The desired constituent is separated 

 out in a known form, and this is accurately weighed. As a 

 method it is often very complicated, very lengthy, and 

 requires elaborate apparatus and much skill. 



Volumetric method. This consists in submitting the 

 substance to certain characteristic reactions, a measured 

 quantity of a solution of known strength being added 

 until a change looked for occurs. From the quantity of 

 reagent used, the amount of the substance found can be 

 calculated by known chemical laws. It is less elaborate, 

 much more quickly accomplished, needs simpler apparatus 

 as a rule, is susceptible of great accuracy, and the skill 

 required is less specialized. 



Colorimetric method. This consists in using a reaction 

 which produces a coloured tint, which is compared with 

 the tint obtained from the same treatment of a known 



