486 
PLINT's ITATURAL HISTOET. [Book XXXI. 
lighter than another. There is, however, a more certain mode 
of ascertaining the difference in quality, that water being the 
better of the two which becomes hot and cold with the greatest 
rapidity : in addition to which, not to keep poising a balance, 
after w^ater has been drawn np in vessels, if it is good, it should 
gradually become warmer, they say, when placed upon the 
ground. "Which water, then, of the several kinds will be 
most likely to be good and wholesome ? "Well-water, no doubt, 
if we are to judge from the general use made of it in cities : 
but only in the case of wells in which it is kept in continual 
agitation by repeated drawing, and is refined by the earth acting 
as a filter. These conditions are sufficient to ensure salubrity 
in water : in regard to coolness, the well must be in a shaded 
spot, and the water kept exposed to the air. There is, however, 
one thing above all to be observed, a point, too, of considerable 
importance with reference to the continuance of the flow — the 
spring must issue from the bed of the well, and not from the 
sides. To make water cold to the touch may be efi'ected arti- 
ficially even, either by forcing it to rise aloft or by making it 
fall from a height, and so come in collision with the air, and be- 
come incorporated'^-^ therewith : for in swimming, w^e find, 
w*hen we hold our breath, the water is felt to be all the colder. 
It was the Emperor N^ero's invention^^ to boil water, and 
then enclose it in glass vessels and cool it in snow ; a method 
which ensures all the enjoyment of a cold beverage, without any 
of the inconveniences resulting from the use of snow. Indeed, it 
is generally admitted that all water is more^* wholesome when 
very impure water. Synesius, Ep. xv., gives an account of the " kydros- 
copium" used by the ancients for ascertaining the weight of water. Beck- 
mann enters into a lengthy examination of it, as also an enquiry into the 
question whether the ancients, and among them Pliny, were acquainted 
with the hydrometer. See his Hist. Inv. Vol. II. pp. 163 — 169. Bolins Ed. 
^® Ne manus pendeant.'* These words, which Hardouin pronounces 
to be full of obscurity, have caused considerable discussion. The passage 
appears to be imperfect, but it is not improbable that he alludes to the use 
of the balance or scales for ascertaining the comparative wholesomeness of 
water. " Corripiat." 
^2 .The thread of his reasoning is not very perceptible ; but he seems to 
mean that the more air there is in a body the colder it is. If the air is 
inhaled by a person when eating peppermint, he will be sensible of a cold 
feeling in the mouth. 
13 Galen believes this method to have been known to Hippocrates, and 
Aristotle was undoubtedly acquainted with it. See Beckmann's Hist. Im, 
Yol. II. pp. 143-4. Bohn's Ed, 
This is not at all the opinion at the present day. 
