150 Mayow 



that it results from sulphureous exhalations carried 

 aloft and dispersed through the air ; for that kind of 

 sulphureous matter can only become heated, or con- 

 tribute to the production of heat, by first being kindled. 

 Therefore I think it should be held that the said 

 sultriness is not infrequently the result of a tremulous 

 and unequal movement of the air ; for the air, when 

 agitated by a motion of that kind, becomes very warm 

 as the distinguished Descartes has remarked. For if 

 any one blows vigorously against the back of the hand 

 the breath is felt to be very cold ; while, on the con- 

 trary, if he blows it into the contracted and bent palm 

 of the hand, it becomes not a little warm ; the reason 

 seems to be that when the aerial particles are thrown 

 into a tremulous motipn by being reflected hither and 

 thither in the hollow hand, it happens that the nitro- 

 aerial particles, by their striking against each other^ 

 are gently detached and thrown into the motion that is 

 required for heat. But if a considerable tract of air is 

 at any time agitated by such a motion, it will not only 

 grow warm but will also be in a condition for entering 

 upon a motion of the kind required for the production 

 of lightning. 



As regards the wonderful violence with which 

 lightning sometimes overthrows and burns whatever 

 stands in its way, the distinguished Gassendi very in- 

 geniously supposes that glomeres (as he calls them) 

 consisting of nitrous, vitriolic, and sulphureous ex- 

 halations, together with a small portion of cloud 

 gathered round them, descend to the earth, and that^ 

 when they take fire at last, they burst into a very im- 

 petuous flame and destroy everything they come near. 

 But, indeed, not to say that nitrous or vitriolic vapours 

 do not exist in the air (as I endeavoured to show above), 

 it is scarcely probable that a small portion of cloud (if 



