510 The Book of the Balance of Wisdom . [October, 
and reservoirs, in respeft to rarity and density, together with 
the incidental difference due to the variety of seasons and 
uses. So then the water of some determined region and 
known city is selected, and we observe upon the water-weight 
of the body, noting exactly what it is, relatively to the 
weight of ioo mithkals ; and we refer [all] operations to 
that [result as a standard] , and keep it in mind against the 
time when we are called upon to perform them, if the 
Supreme God so wills. In winter one must operate with 
tepid, not very cold water, on account of the inspissation 
and opposition to gravity of the latter, in consequence of 
which the water-weight of the body [weighed in it] comes 
out less than it is found to be in summer. This is the 
reason why the water-bowl settles down when the water has 
just the right degree of coldness, and is in slow motion, 
while in case it is hot and moving quickly, or of a lower 
temperature, yet warmer than it should be, the bowl does 
not settle down as when the water is tepid. The temperature 
of water is plainly indicated, both in winter and summer; let 
these particulars therefore be kept in mind.”— (LeCt. V., 
Chap, vi., § 5.) 
An examination of these extracts compels a belief that 
the Arabian philosophers of the twelfth century knew the 
air to have weight, though they never applied the means 
they had discovered of measuring it. The sentence in 
italics leads to the conjecture that they also had some means 
of determining the temperature of water ; possibly a form 
of aerometer was the instrument employed, and they were 
thus enabled to recognise the faCt that the density of water 
increases in proportion to its coldness. 
Al-Khazini’s work contains several tables of the specific 
gravities of substances determined either by the Balance of 
Wisdom or by the hydrometer of Pappus. In these tables 
are enumerated fifty substances, nine of which are metals, 
ten precious stones, thirteen materials of which models 
were made, and eighteen liquids. The smallness of the list 
is not surprising, for most of the substances contained in 
modern lists of specific gravities were entirely unknown to 
the Arabians ; the exactness of the results obtained is mar- 
vellous, when we take into consideration the coarseness of 
their means of graduating instruments and the backward 
state of the mechanical arts at that period. 
The first table comprises the specific gravities of seven 
metals and two alloys ; the results, interpreted into our 
system, together with the values assigned by modern authori- 
