460 Analyses of Books. [Dec. 



vapour increases with its temperature. According to him, this 

 capacity is doubled for every 27 degrees of heat added to air. 

 Air at the temperature of 32° is capable of holding -p^th of its 

 weight of vapour, at 59° it becomes capable of holding ^Lth 

 part, at 86° ^th part, at 1 13° J^th part, and at U0° T Vth 

 part. These estimates do not agree with those previously given 

 by Mr. Dalton, from whose table it appears that the quantity of 

 vapour which air can contain increases at a greater rate than Mr. 

 Leslie makes it do. 



Mr. Leslie adopts the old opinion respecting the state of 

 vapour in the atmosphere. He conceives that it is held in solu- 

 tion by the air precisely as salt is by water. He docs not inform 

 us of the reasons that induce him to adopt this opinion : but the 

 result of his own experiments, if he had attended to it, would 

 have shown him that the supposed analogy between the solvent 

 power of water and air does not in reality exist. The solvent 

 power of air, he says, is doubled by every additional 21 degrees 

 of heat. But can any case be produced in which the solvent 

 power of water increases in any similar proportion ? I believe 

 not. 



Mr. Leslie in this part of his book makes us acquainted with 

 a number of instruments which he has invented for different 

 purposes. The great principle upon which these instruments act 

 is in most cases the same, though the information which they 

 convey be different. This is probably the reason why they have 

 been described all together. It would have been better if each 

 instrument had been described in a separate chapter, or at least 

 a separate section. These instruments are, 



1. The Hygrometer. — This instrument is merely the differen- 

 tial thermometer, having one ball covered with bibulous paper 

 and silk, and the other made of coloured glass. The paper is 

 moistened. The evaporation of this water produces cold, and 

 this cold increases with the rapidity of the evaporation. Now 

 this rapidity depends upon the comparative dryness of the sur- 

 rounding air. Hence the lower the liquid falls in the tube of 

 the hygrometer, or the greater cold produced, the drier is the 

 air. Mr. Leslie gives us a curious collection of facts obtained 

 by his observations with this instrument. 



2. The Pyroscope. — This is an instrument for measuring the 

 intensity of a fire. It is merely the differential thermometer 

 with one of its balls covered with silver leaf, while the other is 

 naked. The fire heats the naked ball, but not the silvered ball. 

 Hence the liquid in the tube rises or falls according to the inten- 

 sity of the fire, and of course marks that intensity. 



3. The Photometer. — This is the differential thermometer, 

 having one of its balls of colourless glass, the other of black 

 glass. The light produces no effect upon the transparent ball. 



