1869.] On the Behaviour of Thermometers in a Vacuum. S19 



experiments ; the case of toluol may be taken as representative of a great 

 number of others. The specific gravity of this liquid is 0*85, that of 

 water being unity; the specific gravity of its vapour is 3*26, that of 

 aqueous vapour being 0'6. Now, as the size of the cloud-particle is 

 directly proportional to the specific gravity of the vapour, and inversely 

 proportional to the specific gravity of the liquid, an easy calculation proves 

 that, assuming the size of the vapour polyhedra in both cases to be the 

 same, the size of the particle of toluol cloud must be more than six times 

 that of the particle of aqueous cloud. It is probably impossible to test this 

 question with numerical accuracy ; but the comparative coarseness of the 

 toluol cloud is strikingly manifest to the naked eye. The case is, as I have 

 said, representative. 



In fact, aqueous vapour is without a parallel in these particulars ; it is 

 not only the lightest of all vapours, in the common acceptation of that 

 term, but the lightest of all gases except hydrogen and ammonia. To this 

 circumstance the soft and tender beauty of the clouds of our atmosphere is 

 mainly to be ascribed. 



The sphericity of the cloud-particles may be immediately inferred from 

 their deportment under the luminous beams. The light which they shed 

 when spherical is continuous : but clouds may also be precipitated in solid 

 flakes ; and then the incessant sparkling of the cloud shows that its particles 

 are plates, and not spheres. Some portions of the same cloud may be com- 

 posed of spherical particles, others of flakes, the difference being at once 

 manifested through the calmness of the one portion of the cloud, and the 

 uneasiness of the other. The sparkling of such flakes reminded me of the 

 plates of mica in the River Rhone at its entrance into the lake of Geneva, 

 when shone upon by a strong sun. 



III. " On the Behaviour of Thermometers in a Vacuum." By 

 Benjamin Loewy, F.R.A.S. Communicated by Prof. Stokes, 

 Sec. R.S. Received January 8, 1869. 



1. In the year 1828 General Sabine made a series of pendulum-experi- 

 ments* in a receiver from which the air was exhausted, and observed inci- 

 dentally that on the pump being worked the thermometer in the receiver 

 fell about 7-tenths of a degree of Fahrenheit's scale when the pressure was 

 reduced to 7 inches, while the converse took place when the air was re- 

 admitted. He ascribed this effect to the removal of the pressure of the 

 atmosphere on the exterior of the bulb and tube of the thermometer ; and 

 to ascertain whether this explanation was correct the following experiment 

 was made : — A thermometer being immersed in pounded ice and placed on 

 the brass plate of an air-pump, the mercury coincided exactly with the 

 division of 32°; it was then covered with a receiver, and the air with- 

 drawn; the thermometer fell as the pump was worked, and when the 

 * Published in the Philosophical Transactions, 1829, part 1. 



