BOOK II.] INSTANCES OF THE CROSS. 513 



masses are upheaved, and such weights discharged as we observe 

 in large mines and mortars, there are two cross-ways before us 

 with regard to this nature. This motion is excited either by the 

 mere effort of the body expanding itself when inflamed, or by 

 the assisting effort of the crude spirit, which escapes rapidly 

 from fire, and bursts violently from the surrounding flame as 

 from a prison. The school, however, and common opinion only 

 vonsider the first effort ; for men think that they are great phi 

 losophers when they assert that flame, from the form of tho 

 element, is endowed with a kind of necessity of occupying a 

 greater space than the same body had occupied when in the 

 form 01 powder, and that thence proceeds the motion in ques 

 tion. In the mean time they do not observe, that although this 

 may be true, on the supposition of flame being generated, yet 

 the generation may be impeded by a weight of sufficient force to 

 compress and suffocate it, so that no such necessity exists as 

 they assert. They are right, indeed, in imagining that the ex 

 pansion and the consequent emission or removal or the opposing 

 nody, is necessary if flame be once generated, but such a neces 

 sity is avoided if the solid opposing mass suppress the flame 

 before it be generated ; and we in fact see that name, especially 

 at the moment of its generation, is mild and gentle, and requires 

 a hollow space where it can play and try its force. The great 

 violence of the effect, therefore, cannot be attributed to this 

 cause ; but the truth is, that the generation of these exploding 

 flames and fiery blasts arises from the conflict of two bodies of a 

 decidedly opposite nature, the one very inflammable, as is the 

 sulphur, the other having an antipathy to flame, namely, the 

 crude spirit of the nitre ; so that an extraordinary conflict takes 

 place whilst the sulphur is becoming inflamed as far as it can 

 (for the third body, the willow charcoal, merely incorporates and 

 conveniently unites the two others), and the spirit of nitre is 

 escaping, as far also as it can, and at the same time expanding 

 itself (for air, and all crude substances, and water are expanded 

 by heat), fanning thus, in every direction, the flame of the sul 

 phur by its escape and violence, just as if by invisible bellows. 



Two kinds of instances of the cross might here be used, the 

 one of very inflammable substances, such as sulphur and cam- 

 phire, naphtha and the like, and their compounds, which take tire 

 more readily and easily than gunpowder if left to themselves 

 (and this shows that the effort to catch fire does not of itself 

 produce such a prodigious effect) ; the other of substances which 

 avoid and repel flame, such as all salts ; for we see that when 

 they are cast into the fire, the aqueous spirit escapes with a 

 crackling noise before flame is produced, which also happens ia 

 n loss degree in stiff leaves, from the escape of the aqueous part 

 before the oily part has caught fire. This is more particularly 

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