134 CORRELATION OF PHYSICAL FORCES. 



An objection to which the view I have been advocating is 

 open, and a formidable one, is, the necessity involved in it of 

 an universal plenum ; for if light, heat, electricity, &c., be affec- 

 tions of ordinary matter, then matter must be supposed to be 

 everywhere where these phenomena are apparent, and con- 

 sequently there can be no vacuum. 



These forces are transmitted through what are called 

 vacua, or through the interplanetary spaces, where matter, if 

 it exist, must be in a highly attenuated state. 



It may be safely stated that hitherto all attempts at pro- 

 curing a perfect vacuum have failed. The ordinary air- 

 pump gives us only highly rarefied air ; and, by the principle 

 of construction, even of the best, the operation depends upon 

 the indefinite expansion of the volume of air in the receiver ; 

 even in the vacuum which is formed in this, so great is the 

 tendency of matter to fill up space, that I have observed dis- 

 tilled water contained in a vessel within the exhausted receiv- 

 er of a good air-pump has a taste of tallow, derived from the 

 grease, or an essential oil contained in it, which is used to 

 form an air-tight junction between the edges of the receiver 

 and the pump-plate. 



The Torricellian vacuum, or that of the ordinary baro- 

 meter, is filled with the vapour of mercury ; but it might be 

 worth the trouble to ascertain what would be the effect of a 

 good Torricellian vacuum, when the mercury in the tube is 

 frozen, which might, without much difficulty, be now effected 

 by the use of solid carbonic acid and ether ; the only proba- 

 ble difficulty would be the different rates of contraction of 

 mercury and glass, at such a degree of cold, and more par- 

 ticularly the contraction of mercury at the period of its 

 solidification. Davy, however, endeavoured to form a 

 vacuum, in a somewhat similar manner, over fused tin, with 

 but partial success ; he also made many other attempts to' 

 obtain a perfect vacuum ; his main object being to ascertain 

 what would be the effect of electricity across empty space : 



