Ill 



ing, harmful and unfit for use. The presence of large quantities 

 of common salt may also indicate animal contamination, but this 

 is only to be taken as a guide when it occurs with excess of 

 Nitrogen compounds, for, though the amount of common salt (or 

 Sodium Chloride) is somewhat above the average in the Dover 

 water, this must be ascribed to our proximity to the sea, whence it 

 sometimes happens that infiltration of sea water into the chalk 

 reseiToirs takes place, and also the salt spray is mechanically 

 carried over the land, and the salt is dissolved by the next shower 

 of rain. 



By means of a standard solution of silver nitrate I will shew 

 you the method of determining the amount of chlorides in drinking 

 water. The method depends upon the fact, that though silver 

 nitrate produces a reddish brown precipitate with chromate of 

 potash, this precipitate does not appear in the presence of a chloride 

 till all the latter has been thrown down as silver chloride. Then 

 the next drop of silver nitrate solution produces a reddish tinge. 

 We shall find one result gives a little over three grains of salt per 

 gallon. This agrees very fairly with the results of Drs. "Wigner 

 and Glaister. The result of Dr. Letheby is evidently too low, but 

 the amount of salt varies considerably with the amount of rainfall. 



We will now discuss the effect produced on the rain water by 

 its coming in contact with decaying vegetable matter. As we 

 know, tlie element carbon is the essential constituent of vegetable 

 matter, and hence, in decaying, carbonic acid gas is produced. 

 This gas is soluble to a large extent in water, and so the rain water 

 absorbs it. Water containing carbonic acid in solution acquires the 

 property of being able to dissolve chalk, forming with it a 

 bicarbonate of lime. Chalk does not dissolve in water free from 

 carbonic acid to any appreciable extent. Water containing chalk 

 in solution is perfectly clear to look at, but as soon as the carbonic 

 acid gas is got rid of, either by boiling, or by removiag it in some 

 other way, the chalk, being unable to remain in solution without 

 the presence of carbonic acid gas, is precipitated. Hence the "fur" 

 in kettles and hot water pipes which causes grave inconveniences to 

 householders. Such water, from the difficulty of getting a lather 

 with soap when it is used for washing purposes, is called " hard 

 water " ; but as the water can be softened by boiling, as above 

 indicated, it is called " temporary hard water" to distinguish it 

 fi'om the water containing Sulphate of Lime (Plaster of Paris) or 

 Sulphate of Magnesia (Epsom Salts) in solution. Water in the 

 Trent district is hard from the presence of these salts and is not 

 softened by boiling. Such water is known as permanently hard 

 water. In the Dover water we have some 12° or thereabouts 

 of temporary hardness, that is 12 grains of chalk per gallon and 

 3" or so of hardness due to the presence of these sulphates. This 



