BOOK U.] METHOD OP ANALYSIS. 523 



by the thermometer, as we described it above ;* for the cold and 

 heat are not actually perceived by the touch, but heat expands 

 and cold contracts the air. Nor, again, is that expansion or con- 

 traction of the air in itself visible, but the air when expanded 

 depresses the water, and when contracted raises it, which is the 

 first reduction to sight. 



Again, let the required nature be the mixture of bodies ; 

 namely, how much aqueous, oleaginous or spirituous, ashy or 

 salt parts they contain ; or, as a particular example, how much 

 butter, cheese, and whey there is in milk, and the like? These 

 thiniis are rendered sensible by artificial and skilful separations 

 in tangible substances ; and the nature of the spirit in them, 

 though not immediately perceptible, is nevertheless discovered 

 by the various motions and efforts of bodies. And, indeed, in 

 this branch men have laboured hard in distillations and artificial 

 separations, but with little more success than in their other ex- 

 periments now in use ; their methods being mere guesses and 

 blind attempts, and more industrious than intelligent ; and what 

 is worst of all, without any imitation or rivalry of nature, but 

 rather by violent heats and too energetic agents, to the destruc- 

 tion of any delicate conformation, in which principally consist 

 the hidden virtues and sympathies. Nor do men in these sepa- 

 rations ever attend to or observe what we have before pointed 

 out ; namely, that in attacking bodies by fire, or other methods, 

 many qualities are superinduced by the fire itself, and the other 

 bodies used to effect the separation, which were not originally in 

 the compound. Hence arise most extraordinary fallacies ; for 

 the mass of vapour which is emitted from water by fire, for 

 instance, did not exist as vapour or air in the water, but is chiefly 

 created by the expansion of the water by the heat of the fire. 



So, in general, all delicate experiments on natural or artificial 

 bodies, by which the genuine are distinguished from the adul- 

 terated, and the better from the more common, should be 

 referred to this division; for they bring that which is not the 

 object of the senses within their sphere. They are therefore 

 to be everywhere diligently sought after. 



With regard to the fifth cause of objects escaping our senses, 

 it is clear that the action of the sense takes place by motion, 

 and this motion is time. If, therefore, the motion of any body 

 be either so slow or so swift as not to be proportioned to the 

 necessary momentum which operates on the senses, the object 

 is not perceived at all ; as in the motion of the hour hand, and 

 that, again, of a musket-ball. The motion which is imperceptible 

 by the senses from its slowness, is readily and usually rendered 



■ Sea Table of Degrees, No. 38. 



