34 
POPULAR SCIENCE REVIEW. 
to be obtained with great difficulty, and so far it is fortunate that 
it is not to be held as at all requisite for maintaining health. All 
that is wanted, and this is what we mean by a “ pure drinking- 
water,” is water free from everything of ascertained or suspected 
power to induce ill-health, and — we add this as a corollary only 
— from everything unpleasant to the senses and the imagination. 
Consequently, guided by general experience and medical autho- 
rity, the presence in small quantity of chlorides of sodium, of 
calcium, or of magnesium, carbonate of calcium (chalk) or of 
magnesium, or sulphate of calcium or of magnesium, either 
alone or together, does not deprive a water of this title. The 
presence, too, of certain gases — carbonic acid, oxygen and nitro- 
gen — leaves the water still pure for drinking purposes. 
Certain other substances, harmless even for some time to those 
drinking water containing them in small quantity, and even health - 
restoring to others, render a water notwithstanding unfit to rank 
as a pure drinking-water. Take the example of hepatic waters 
(waters containing sulphuretted hydrogen) and that of chalybeate 
waters (waters containing combinations of iron) : however useful 
these waters may prove to a valetudinarian, or harmless to any 
one for a time, they are found to be hurtful on protracted use. 
Other substances, again, which seem to have in themselves 
nothing baneful, hardly justify a drinking-water in which they 
are found being denominated pure. The reason of this will be 
sufficiently intelligible. If small quantities of nitrates are found 
in the water, it is in ordinary cases certain that the water has 
passed through matters of animal origin, and therefore is open 
to the likelihood of containing, in some succeeding supply, other 
matters derived from this source highly injurious to health. 
This, again, is the case with ammonia, and for the same reason ; 
but the danger indicated by this substance is'even more imminent, 
as water containing ammonia is almost certain to contain also 
noxious matters of animal origin. All matters immediately de- 
rived from animal* life are objectionable in drinking-water, be- 
cause of our uncertainty as to their exact nature, and therefore as 
to their effects, not to mention the offensiveness of their origin to 
our ideas. And more than this, matters derived from life at all, 
even though this be only vegetable, are to some extent objec- 
tionable — firstly, because we cannot acquit them of all power to 
do harm ; but secondly, and chiefly, because we can hardly obtain 
sufficient guarantee in their presence of the absence of animal 
matters also. 
For though animal matters may be practically looked upon 
as present if with carbonaceous (or organic) matters ammonia 
or nitrates be found also, or if the carbonaceous matter be 
found to be also nitrogenous, there may still be sufficient animal 
