486 plint's natural 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 water has been drawn up 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 fi-om 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 efiected 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, ^^ we find, 

 when we hold our breath, the water is felt to be all the colder. 

 It was the Emperor Nero'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 " hydros- 

 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 

 Avith the hydrometer. See his Eist. Inv. Vol. II. pp. 163 — 169. Bohris 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. ^ n " Corripiat." 



^- -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. 



'3 Galen believes this method to have been known to Hippocrates, and 

 Aristotle was undoubtedly acquainted with it. See Beckmann's Eist. Inv. 

 Vol. II. pp. 143-4. Bohn's Ed. 



^^ This is not at all the opinion at the present day. 



