WATEii SUPPLY AND PUBLIC HEALTH. 
35 
official Reports on the quality of the metropolitan waters are 
frequently published by different chemists, but they are made 
on different plans, and cannot be readily compared with each 
other. 
In old analyses of water the imperial gallon was taken as the 
standard measure, and the results were given in grains per 
gallon. As the gallon contains 70,000 grains of water, this is 
the same as giving parts per 70,000. In 1864 Dr. A. W. 
Hofmann, then of the Royal School of Mines, was appointed to 
report on the metropolitan waters to the Registrar-Greneral. 
He introduced the system of giving the results of analysis in 
parts per 100,000 — a system which has been continued by his 
successor, Dr. E. Frankland. The analyses for the River 
Pollution and Water Supply Commissions are given in this 
form. The imperial gallon is, however, still retained by most 
chemists ; amongst others, by Dr. Lethebv, Dr. Voelcker, and 
Professor Wanklyn.* 
This want of agreement is frequently a matter of great 
inconvenience. As regards the general constituents of water it 
is less so, because for detailed analyses the standard is generally 
stated. But where “ hardness ” alone is spoken of, it is often 
doubtful which scale is used. This is a point of importance, 
because the term is in frequent use, and we often read of the 
hardness of certain waters when no other chemical facts regard- 
ing them is stated or perhaps known. 
Those who adhere to the gallon as the standard generally 
describe hardness by what is known as Dr. Clark’s scale, which 
is the same as grains per gallon ; a hardness of 7° denoting seven 
grains of carbonate of lime or magnesia (or their equivalent as 
sulphates) in a gallon of water. f Dr. Frankland gives hardness 
in parts per 100,000; so that a hardness of 10° in his analysis 
is only equal to 7° in Clark’s scale. In comparing this and 
other analytical details there is a simple rule:- — to convert parts 
per 100,000 into grains per gallon (or parts per 70,000), multi- 
ply by 7 and divide by 10 ; to convert grains per gallon into 
parts per 100,000, multiply by 10 and divide by 7. 
Chemists are not unanimous as to the best means to be 
* Professor Wanklyn also sometimes states his results in parts per million 
(or millegrammes per litre). A simple division by 10 converts this into 
Dr. Frankland’s scale of parts per 100,000. 
t A further confusion as regards hardness has been introduced by a 
slight modification of Dr. Clark’s method of expressing his result. Even 
distilled water requires to dissolve some soap before a lather is produced ; 
the quantity required is about one grain to a gallon of water. This by some 
chemists is added to the result in order to state the actual soap-absorbing 
power of the water. According to this method, a hardness of 8° is equal to 
seven grains of carbonate of lime per gallon. 
d 2 
