354 NATURE AND MAN. 



distance, we cannot tell by the use of our eyes alone whether the 

 space included by its frame is void, or is occupied by a perfectly 

 transparent and colourless glass. A glass globe is held up in 

 front of it, and we cannot tell by looking at it whether it is empty, 

 or is filled with pure water or some other transparent colourless 

 liquid. And we can take no cognizance by our vision of the 

 atmosphere which surrounds us, unless its transparence is inter- 

 fered with by mist or fog. — Clearly, then, our visual sense cannot 

 per se furnish us with a satisfactory definition of matter.* 



Now that we have got rid of the fiction of " imponderables," 

 we might fall back on a definition of matter — in use before that 

 fiction was invented — as that which possesses "ponderosity" or 

 weight. But what is weight ? The downward tendency, it may 

 be replied, in virtue of which all unsupported bodies fall to the 

 earth. But what is this " tendency ? " We might see any number 

 of bodies falling to the ground, and might frame a correct law of 

 their motion, without having the remotest conception of their 

 possessing that downward pressure, which we at once recognize 

 when we take a lump of lead or iron into our hands ; and it is 

 obviously on our cognition of this pressure, that our idea of 

 weight or ponderosity is based. Now the instrumentality through 

 which we take cognizance of it seems to me to be threefold. In 

 i\-\Q first place, we have the sense of simple pressure on the tactile 

 surface ; as when, the hand passively resting on a table, a weight 

 is laid upon it. Secondly, we recognize it by the sense of tension 

 which we experience when a weight is attached to a pendent limb, 

 and which we refer to the muscles and ligaments which are thus 

 put on the stretch ; or when, the hand resting on the top of a 

 cylinder of glass placed over an air-pump, the air is exhausted 

 from beneath, so as to make us feel the downward " pressure of 



* According to Professor Bain, the conception of space is essentially based 

 on the sense of muscular tension which, according to him, we experience in 

 the ordinary movements of our eyes. But I am satisfied that this is physio- 

 logically erroneous. These movements are ordinarily guided, as Professor 

 Alison long ago contended, and as Professor Ilelmholtz and I myself have 

 since experimentally proved, by the visual, not by the muscular sense ; and it 

 is only when we put the muscles to an unusual strain — as when our visual axes 

 converge on an object brought nearer and nearer to the eyes, or when we 

 entirely exclude light from the retina, that we experience any sense of tension 

 in their muscles. 



