140 SECTION PHYSICS. 



from the same height ; and that would always be the result whatever 

 the force of the discharge. The second, that, on account of the 

 resistance of the air, there is a maximum limit of the velocity with 

 which a body falling from any height can reach the earth. The third, 

 that a motion cannot be destroyed by another motion coming upon it 

 afterwards ; and this was illustrated by the well-known experiment of 

 a cannon mounted on a waggon drawn by six horses ; when the ball 

 was discharged upwards, it always fell close to the mouth of the 

 cannon, whether the waggon was at a standstill, or whether it 

 was going at full speed. Other experiments refer to the relative 

 weight of air and water ; others to the impermeability of glass to 

 moisture and smells ; some are directed, in accordance with one of 

 Galileo's ideas, in determining whether light takes any time in pro- 

 pagating itself; others again clearly demonstrate that even white 

 substances can be set on fire by means of a powerful burning-glass j 

 nor did theyfail to observe thephenomena called byus phosphorescence 

 in sugar, in rock salt, rock-crystal, in agates, in jasper, &c. 



Chemistry, likewise, was included in the field of their researches : 

 they observed that waters distilled in lead, render turbid all natural 

 waters, which are afterwards made clear by means of strong vinegar 

 (acetoforte) ; that oil of tartar and oil of aniseed infused in water 

 occasion the formation of a little cloud, which disappears on the 

 addition of spirits of sulphur ; and they insist strongly on the fact that 

 the thickness is less with purer and lighter waters, and hence point 

 out the above-mentioned liquids for the analysis of waters ; that the 

 tincture of roses, extracted with oil of vitriol, becomes green when oil 

 of tartar is poured upon it, and returns to its original colour by means 

 of spirits of sulphur ; that citric acid, spirits of vitriol, and spirits of 

 sulphur change lac into violet, and the tincture of violets into ver- 

 milion, which can be made violet by the addition of oil of tartar, &c. 

 But there remains to tell of more interesting experiments namely, of 

 those made on artificial freezing, and on the dilation of bodies by 

 heat. It was Galileo's opinion that ice was rather rarefied water than 

 condensed water; since condensation brings about diminution of 

 bulk and increase of weight, whilst rarefaction, on the other hand, 

 causes greater lightness and enlarges the bulk. But water, in freezing, 

 augments in bulk, and ice already made, is lighter than the water 



