LIGHT. 119 



Comte thus expresses himself on this subject: 'Mathema- 

 ticians, too frequently taking the means for the end, have 

 embarrassed natural philosophy with a crowd of analytical 

 labours founded upon hypotheses extremely hazardous, or even 

 upon conceptions purely visionary, and consequently sober- 

 minded people can see in them nothing more than simple 

 mathematical exercises, of which the abstract value is some- 

 times very striking, without their influence in the slightest 

 degree accelerating the natural progress of physics.' 



An objection to which the view I have been advocating is 

 open, and at first sight a formidable one, is the necessity 

 involved in it of an universal plenum ; for if light, heat, elec- 

 tricity, &c., be affections of ordinary matter, then matter must 

 be supposed to be everywhere where these phenomena are 

 apparent, and consequently there can be no vacuum. 



These forces are transmitted through what are called vacua, 

 or through 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 distilled water 

 contained in a vessel within the exhausted receiver 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 pro- 

 bable difficulty would be the different rates of contraction of 

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

 larly the contraction of mercury at the period of its solidifica- 

 tion. Davy, indeed, endeavoured to form a vacuum, in a 



