﻿Physics and Chemistry. 73 



of the inner and outer cylinders respectively. Whence 



™* R 



Fdz = . K and V 2 = 4F log — 

 4 log - & ?• 



By measuring R and r in centimeters and F in dynes, V will 

 be obtained in absolute C. G. S. units. To measure F, known 

 weights are placed in a small scale pan above the suspended cyl- 

 inder until equilibrium is obtained. This value in grams is con- 

 verted into dynes by multiplying by g. The oscillations of the 

 balance-beam are damped by means of a cardboard disk attached 

 to it in place of the second pan and movable within a slightly 

 larger cylindrical vessel of glass. Since within certain limits the 

 force is independent of the position of the inner cylinder, a mir- 

 ror may be attached above the knife edge and the displacements 

 read with a telescope and scale ; their value in dynes being ob- 

 tained by multiplying the constant of displacement by the tangent 

 of the inclination angle. This constant is determined experi- 

 mentally once for all, by noting the angular displacement pro- 

 duced by known weights when the electrometer is not charged. 

 Calculation shows that in the actual instrument as constructed, a 

 want of coincidence of 3 mm between the axes of the cylinders 

 changes F by only 0*003 of its value. A slight correction may 

 be made for the stem of the inner cylinder. The results given by 

 the apparatus are satisfactory. — C. i2., cii, 753-756, March, 1886. 



G. F. B. 



2. On the Influence of Temperature on the Heat of Chemical 

 Combination. — Because of the rudimentary character of our 

 knowledge of the influence exerted by temperature upon the heat 

 of chemical combination, Pickering has undertaken to investi- 

 gate it, by measuring this heat of combination within the limits 

 of ordinary atmospheric temperatures, about 25°. Since hydrated 

 salts afford many instances of small energy of combination, and 

 since such substances would be most likely to exhibit a variation 

 in the heat of their formation within these narrow limits, these 

 salts were chosen for the investigation. Whenever a salt is dis- 

 solved in water, the difference between the heat of solution in 

 the anhydrous and hydrated states gives the heat of combination 

 of the salt with its water of crystallization, provided the heat 

 absorbed in the conversion of the solid water into liquid water 

 in the second case, is subtracted from the latter. The author finds 

 " that the relation between the constituent portions of the mole- 

 cule of a complex solid undergoes a series of modifications as the 

 temperature changes, the result of which is that the variations 

 exhibited in the heat of combination, and consequently in the 

 specific heat also, being conditioned by a different order of circum- 

 stances at different temperatures, exhibit irregularities of a very 

 marked character." The summary and conclusions of this re- 

 search we give in his own words: (1) The heat of dissolution of 

 a salt at different temperatures is not represented by a straight 



