PHYSICS. 6bo 



two upper plates, liaving cop])er stoppers, the intervals between the 

 plates maybe filled with fluid. Air was first used, and the author 

 proved that its conductivity for heat increases with the temperature. 

 The ratio of the conductivity of air to that of liquids was studied, the 

 liquid being placed in the lower space. Experiments were also made 

 with dry and wet plate glass and also with marble. The author i)oints 

 out the applicability of his apparatus to measure resistances, the po- 

 tential being measured instead of temperature. [Nature, October, 1881, 

 xxiv, 593.) 



Crookes has made a series of experiments on the conduction of heat 

 in highly rarefied air. An accurate thermometer with a pretty open 

 scale was inclosed in a 1 J-inch glass globe, the bulb of the thermometer 

 being in the center and the stem being inclosed in the tube leading 

 from the globe to the pump. The globe was brought to a uniform tem- 

 perature in a vessel of water at 25"^ and was then suddenly j^lunged 

 into a large vessel of water at 65°. The number of seconds required 

 for the thermometer to rise from 25° to 50° was recorded. At 700 mil- 

 limeters pressure, 121 seconds was required ; at 1 millimeter, 150 sec- 

 onds; at 620 M (millionths of an atmosphere), 102 seconds ; at 117 M, 

 183 seconds ; at 59 M, 203 seconds ; at 23 M, 227 seconds ; at 12 M, 252 

 seconds ; at 5 M, 322 seconds ; and at 2 M, 412 seconds. Hence there is 

 not only a notable diminution in the rate at which heat is conveyed across 

 the space in the bulb, but the reduction of pressure from 5 M to 2 jVI 

 produces twice as much retardation in the rate as is obtained by the 

 whole exhaustion from 700 millimeters to 1 millimeter. The author 

 thinks, therefore, that in such vacua as exist in planetary space the loss 

 of heat would be exceedingly slow. {Nature, January, 1881, 234, xxiii.) 



Langley has devised an apparatus for the detection and measurement 

 of radiant heat, which is a thousand times more sensitive than the ther- 

 mopile, and which is capable of indicating a change in temperature of 

 ^ 0^0 of a degree centigrade. He calls it an actinic balance or bo- 

 lometer. It consists of two disks of ebonite, 30 millimeters diameter and 

 3 millimeters thick, each with a concentric opening in the center 8 milli- 

 meters square. On the face of each disk strip9«of iron a little less than 

 0.5 millimeter wide and about 0.004 millimeter thick cross the opening 

 like a grating, those on one disk coming opposite the intervals in the 

 other. These twenty-nine strips, of which fifteen are on one disk and 

 fourteen on the other, are arranged in two sets, fifteen in one and 

 fourteen in the other. The first set, eight on one disk and seven on the 

 other, are placed centrally ; the second set is divided, one-half being on 

 each side of the other set. The strips in each set are all connected 

 together in series so that an electric current would traverse them suc- 

 cessively. The two disks are fastened together and placed in a hollow 

 cylinder of ebonite, lined with copper, and provided with suitable dia- 

 phragms. Each of the sets of strips is made one side of a Wheatstone's 



