44 PHIXCIL'LES OF CHEMISTRY 



contains various substances in solution, mostly salts that is, sub- 

 stances resembling common table salt in their physical properties and 



than 5 grams. The water of the Seine contains about 15 grams of nitrates, and the Rhone 

 about 8 grams. The amount of ammonia is much less ; thus in the water of the Rhine 

 about 0*5 gram in June, and 0'2 gram in October ; the water of * he Seine contains the 

 same amount. This is less than in rain water. Notwithstanding this insignificant 

 quantity, the water of the Rhine alone, which is not so very large a river, carries U'>.'_!4.1 

 kilograms of ammonia into the ocean every day. The difference between the amount oi 

 ammonia in rain and river water depends on the fact that the soil through which tht 

 rain water passes is able to withhold the ammonia. (Soil can also absorb many othei 

 substances, such as phosphoric acid, potassium salts, Arc.) 



The water of springs, rivers, wells, and in general of those localities from which it is 

 taken for drinking purposes, may be very injurious to the health if it contains much 

 organic pollution all the more, as in such water the lower organisms (bacteria) maj 

 rapidly develop, and these organisms often serve as the carriers or causes of infectious- 

 diseases. Thanks to the work of Pasteur, Koch, and many others, this province of researcl 

 has made considerable progress during the past ten years, and has shown the possi- 

 bility of investigating even the number and properties of the germs held by water 

 because those pathogenic bacteria which produce sickness, such as typhoid fever, Siberiai 

 plague, &c., have been distinguished. In bacteriological researches, a gelatinous 

 medium, enabling the germs to develop and multiply, is prepared with gelatin and water 

 which has previously been heated several times, at intervals, to 100 (it is thus renderec 

 sterile that is to say, all the germs in it are killed). The water to be investigated 

 is added to this prepared medium in a definite and small quantity (it is sometimes 

 diluted with sterilised water to facilitate the calculation of the number of germs), it is 

 protected from dust (which contains germs), and is left at rest until whole families o: 

 lower organisms are developed from each germ. These families (colonies) are visible tc 

 the naked eye (as spots), they may be counted, and by examining them under th< 

 microscope and observing the number of organisms they produce, their significance ma> 

 be determined. The majority of bacteria are harmless, but there decidedly are patho 

 genie bacteria whose presence is one of the causes of malady, and of the spreading o 

 certain diseases. The number of bacteria in one cubic centimetre of water sometime! 

 attains the immense figures of hundreds of thousands and millions. Certain well, spring 

 and river waters contain very few bacteria, and are free from disease-producing bacterii 

 under ordinary circumstances. By boiling water, the bacteria in it are killed, but th< 

 organic matter necessary for their nourishment remains in the water. The best kind: 

 of water for drinking purposes do not contain more than 800 bacteria in a cnbii 

 centimetre. 



The presence in water of every residue of destroyed organisms may be partly judgc< 

 from the amount of combined nitrogen, as all organisms contain nitrogen compotmdfl 

 It is mo'st essential to distinguish and determine nitrogen in the form of organic mattei 

 and in the form of oxides (nitric acid). The former is not separated, on heating, iron 

 water by the action of reducing agents, such as sulphurous anyhdride, whilst thi 

 nitrogen which occurs as oxide is evolved by this means. Thus on adding hydrochlori 

 "acid and ferrous chloride to water, the nitrogen of the nitric acid gives oxide of nitrogen 

 which may be determined. The presence of nitric acid indicates that the organr 

 matter in water has already been oxidised. Water which contains more than 1 par 

 of nitrogen (in this form) in a million parts is considered as injurious, and should no 

 be used. Frankland found about r.s parts of nitrogen in an oxidised form, and Iron 

 0'22 to 0*5 part in organic combinations in the water of the Thames at London. 



The amount of gases dissolved in river water is much more constant tha 

 solid constituents. One litre, or 1,000 c.c., of water contains 40 to 5,1 

 measured at normal temperature arid pressure. In winter the amount of <ra 

 than in summer or autumn. Allowing that a litre contains 50 c.c. of gases, it may b 

 admitted that these consist, on the average, of 20 vols. of nitrogen, 20 vols. of carl ion i 



