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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 vapor which is emitted from water by fire, for 

 instance, did not exist as vapor 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 re- 

 ferred to this division ; for they bring that which is not the ob- 

 ject 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 impercepti- 

 ble by the senses from its slowness, is readily and usually ren- 

 dered sensible by the accumulation of motion; that which is 

 imperceptible from its velocity, has not as yet been well meas- 

 ured; it is necessary, however, that this should be done in 

 some cases, with a view to a proper investigation of nature. 



The sixth case, where the sense is impeded by the power of 

 the object, admits of a reduction to the sensible sphere, either 

 by removing the object to a greater distance, or by deadening 

 its effects by the interposition of a medium, which may weaken 

 and not destroy the object ; or by the admission of its reflection 

 where the direct impression is too strong, as that of the sun in 

 a basin of water. 



The seventh case, where the senses are so overcharged with 

 the object as to leave no further room, scarcely occurs except in 

 the smell or taste, and is not of much consequence as regards 

 our present subject. Let what we have said, therefore, suffice 

 with regard to the reduction to the sensible sphere of objects 

 not naturally within its compass. 



Sometimes, however, this reduction is not extended to the 

 senses of man, but to those of some other animal, whose senses, 

 in some points, exceed those of man ; as (with regard to some 

 scents) to that of the dog, and with regard to light existing 

 imperceptibly in the air, when not illuminated from any ex- 

 traneous source, to the sense of the cat, the owl, and other 

 animals which see by night. For Telesius has well observed, 

 that there appears to be an original portion of light even in the 



