380 Dr. Gray’s Account of the Earthquake 
I have ventured to propose the consideration of these mixed 
causes,* if I may so term them, because I think it very cer- 
tain that none of the theories hitherto formed respecting 
earthquakes, are by any means adequate to explain their va- 
rious phenomena. I leave it to future theorists, should any 
adopt the principle, to determine which of the elements is 
concerned in a primary, and which in a secondary way ; also 
what share in the attendant phsenomena is to be assigned to 
each element. 
In whatever light the subject is viewed it seems to present 
so many difficulties, that perhaps the best we can do is, to 
consider our knowledge respecting earthquakes as consisting 
merely in a certain number of facts, to which many more must 
probably be added, before we shall be able to understand the 
cause of a phenomenon whose dreadful effects have, in all ages, 
been too well known in every quarter of the world. So fre- 
quent indeed are earthquakes in some parts, that it is impos- 
sible, without being fully sensible of our happiness, to reflect 
* Dr. Hales’s theory of earthquakes was founded upon a sort of mixed cause: he 
supposed sulphureous vapours to arise from the earth, and to form clouds, the explo- 
sive lightning of which kindled the ascending vapours in the earth, and thereby caused 
what he calls an earth lightning ; which lightning he considered as the immediate 
eause of earthquakes. Yet, so imperfect was the aerial chemistry of those days, that 
the experiment by which he illustrates the operation of his sulphureous vapours, con- 
sists only in mixing together nitrous gas and common air, to show the red fumes, and 
diminution of bulk produced. (See Phil. Trans. Vol. XLVI. page 672 and 677.) If 
Dr.HALES had been acquainted with modern chemistry, he would undoubtedly,! think, 
have made inflammable gas the basis of his theory; and indeed, when it is considered 
that there are great quantities of this gas in the earth, that it readily ascends into the 
atmosphere, and that, when inflamed, it is capable of producing the most violent ef- 
fects, it appears to me that it has at least as much claim as the electric fluid, to be rec- 
koned among the probable agents in earthquakes. 
