200 Sir G. S. Mackenzie, Bart., on the 



other, What is the source from which it is evolved % These 

 questions are now put on the assumption, that motion can- 

 not be identical with sound, as supposed in Sir J. Herschel's 

 definition, though motion be necessary to the evolvement of 

 sound. Sound, therefore, I consider as a thing sui generis ; 

 as hght, heat, and electricity.* 



I consider farther, that it is contained abundantly in atmo- 

 spheric air, and more or less, in proportion to density, in aeri- 

 form or gaseous matter ; and this, perhaps, exclusively. It 

 can scarcely be objected, that gaseous matter cannot contain, 

 or be combined with anything, but what is proper to its 

 own constitution, or, more properly speaking perhaps, to its 

 proper nature ; for we know, that it owes its condition to 

 heat ; and that common air, when suddenly compressed, 

 gives it out so abundantly as to set fire to combustible bodies. 

 When the electric spark passes from a body in which it is 

 concealed, through the air towards another body, light is 

 given out. We are not, indeed, in a condition to say which 

 gives out the light, the electric fluid or the air. Light, how- 

 ever, evidently exists in the one or the other, or in both. No 

 doubt, in an exhausted receiver we see light emanating from 

 the electric fluid dispersed through the vacuum ; but the in- 

 tensity is very far below that of the spark passing through 

 the air. From this we may, at least, infer, that the air is 

 necessary to the brilliancy of the spark. Experiments have 

 been made to shew, that different gases have different powers 

 of conduction in reference to sound. The results, however, 

 in respect to my views, shew that diff'erent gases contain dif- 

 ferent quantities of sound. Other questions now suggest 

 themselves. 



* There is sometimes a want of precision in the mode of expression made 

 use of by some writers on sound, which contributes to the difficulty of the sub- 

 ject. For instance, at the outset of the article. Acoustics, in the Encyclopaedia 

 Britannica, it is said, " The report of a cannon, for example, produces a con- 

 cussion which shakes the ground under, and to a great distance around it." It 

 is evident, that, in this case, the cause of the concussion is not, properly speak- 

 ing, the report, but the force elicited by the sudden liberation of highly expan- 

 sive substances from confinement in the gunpowder. If a cannon could be 

 fired in a vacuum, no report would be heard, but the concus,no7i would be 

 equally felt. 



