336 On the Lgniting Point of Petroleum. 
but that is possessed by undoubted plants, so chat is not 
conclusive. On the other hand, it is asserted that they 
resemble plants in decomposing carbonic acid and libera- 
ting oxygen, whilst animals do the contrary. This to me 
seems to settle the question, but everyone had better judge 
for himself. Again, the reader may say, “ You tell me of 
things that are covered with round dots, what are those 
dots ? raised knobs, little pits, or only surface markings?” 
This, too, like most things connected with them, is a bone 
of contention. Some will have it that they are prominences, 
others depressions ; but my impression is, that it is some- 
times one, and sometimes the other; for some break in such 
a manner as to lead to the idea of their being indentations, 
whilst others break in the contrary direction. And if you 
happen to get some of them on their edges and look along 
their surfaces, some exhibit spikes, others not; and the 
mode in which shadows fall when they are viewed by 
oblique light leads now to one and then to the other con- 
clusion. 
Finally, I may state that the reason I have made so 
many comparisons is, that the reader may recognize the 
forms when found.—Jézd. 
ON THE IGNITING-POINT OF PETROLEUM. 
BY JOHN ATTFIELD, PH.D., F.C.S. 
Director of the Laboratory of the Pharmaceutical Society of 
Great Britain. 
t is now well known that Petroleum, as it issues from 
the earth, and as met with in commerce in the crude 
state, emits a vapour of powerful odour, which is inflam- . 
mable, and which, consequently, when mingled with air 
in certain proportions, forms an explosive mixture. The 
combustible properties of this vapour closely resemble 
those of common coal gas. As with coal-gas, so with petro- 
leum vapour,—a small quantity in a large quantity of air 
gives odour tothe air but does not form an explosive mix- . 
ture. Again, coal-gas, as supplied to the consumer, always 
contains a small percentage of air, and yet the mixture is 
not explosive, so petroleum vapour, even though containing 
