22 PUBLIC HEALTH CHEMISTRY 



The examination should be undertaken as soon as pos- 

 sible, since changes take place on keeping. If delay is 

 unavoidable, changes should be kept at a minimum by 

 packing in ice. 



PHYSICAL EXAMINATION. 



Transparency. 



Suspended matter. 



Colour. Best, bluish or greyish ; greenish, from algae ; 

 yellow or brown suspicious, except peaty. 



Taste. 



Smell. Place 250 c.c. in a glass-stoppered bottle. Put 

 on water-oven at 30 C. for a few minutes. Remove 

 stopper, and smell at once. 



Sediment. Let water stand for a few hours, pipette a 

 few c.c. from bottom, centrifuge, mount a drop on a 

 clean slide, and examine. The deposit may contain a very 

 large number of things. 



1. Mineral matter, such as sand, clay, etc. 



2. Vegetable matter : (a) Living such as bacteria, 



yeasts, moulds, diatoms, desmids, rotiferae ; (b) 

 Dead vegetable cells, husks of grain, cotton or 

 linen fibres, starch granules. 



3. Animal matter : (a) Living such as ova, insects, 



worms, etc. ; (b) Dead such as hairs, scales, 

 muscle fibre. 



CHEMICAL EXAMINATION. 



Reaction. Most drinking waters are alkaline in reac- 

 tion. Upland surface water is often acid from humic and 

 ulmic acids ; and this is important, as these acids dissolve 

 lead. Sewage-contaminated waters usually retain their 

 alkalinity. Waters polluted by refuse from chemical 

 or dye works are sometimes acid in reaction. 



Dissolved Solids. The suspended matter is usually 

 allowed to settle before testing for the solids in solution. 

 The latter are estimated as total, fixed, and volatile. Also 

 note, -when igniting dried solids, presence or absence of 

 fumes, odour, and charring. 



Total Solids. (1) Take a weighed platinum or porcelain 

 dish of sufficient size ; (2) Add 100, 200, 250, 500, or 



