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ON SMELL. 
the six senses — I say six, for the sense of heat must be con- 
sidered to be one. Let us now turn our attention to the others. 
Touch and heat are both recognised by all parts of the body. 
The network of surface nerves, which conveys the sensations to 
the brain, however, is not continuous. It is, perhaps, not quite 
generally recognised that the sense of touch is acute, or perhaps 
I may say experienced at all, only when the finger or other part 
of the body touching is moved over the object touched. Here 
again we have a series of vibrations produced by the inequality 
of the surface touched. We can recognise as rough or smooth, 
surfaces more or less unequal ; but here again the sense of heat 
helps us in recognising objects by this sense. These two are so 
much in union, that it is doubtful whether we ever separate 
them in mental image. We know at once whether we touch 
flannel or linen, because the flannel is a worse conductor of heat 
than the linen, and we are conscious of its not feeling cold. 
Similarly, we could distinguished polished wood from polished 
steel, and, as I have proved experimentally, water from mercury, 
and, I think, from ether and other better conducting liquids. 
The main point I insist on is that the sense of touch is a register 
of vibrations, or, if it is preferred, matter in vibration, if we may 
give a name to what is and ever must be utterly unknown to us, 
the substratum which bear the qualities communicated to our 
senses by vibration. 
The sense of heat may be experienced unaffected by that of 
touch. It is caused by a set of vibrations of the hypothetical 
ether, at a rate slower than that of light. 
Thus a thermometer or thermopile, capable of registering 
very minute changes of temperature, placed in that portion of 
the spectrum beyond the visible red end, shews that the heat 
rays are, like the light rays, refracted by the prism, and that 
they extend for a considerable distance beyond the visible 
spectrum, besides overlapping it. Our skin nerves have the 
