8. _Compounds assumed to be determined by the Mechanical Attraction of the Elements._--When, in consequence of the investigations and discoveries of Newton and his predecessors, the conception of mechanical force had become clear and familiar, so far as the action of external forces upon a body was concerned, it was very natural that the mathematicians who had pursued this train of speculation should attempt to apply the same conception to that mutual action of the internal parts of a body by which they are held together. Newton himself had pointed the way to this attempt. In the Preface to the _Principia_, after speaking of what he has done in calculating the effects of forces upon the planets, satellites, &e., he adds, 'Would it were permitted us to deduce the other phenomena of nature from mechanical principles by the same kind of reasoning. For many things move me to suspect that all these phenomena depend upon certain forces, by which the particles of bodies, through causes not yet known, are either urged towards each other, and cohere according to regular figures, or are repelled and recede from each other; which forces being unknown, philosophers have hitherto made their attempts upon nature in vain.' The same thought is at a later period followed out further in one of the Queries at the end of the Opticks[9\6]. 'Have not the small particles of bodies certain Powers, Virtues, or Forces, by which they act at a distance, not only upon the rays of light for reflecting, refracting and inflecting them, but also upon one another for producing a great part of the phenomena of nature?' And a little further on he proceeds to apply this expressly to chemical changes. 'When Salt of Tartar runs _per deliquium_ [or as we now express it, deliquesces] is not this done by an _attraction_ between the particles of the Salt of Tartar and the particles of the water which float in the air in {11} the form of vapours? And why does not common salt, or saltpetre, or vitriol, run _per deliquium_, but for want of such an attraction? or why does not Salt of Tartar draw more water out of the air than in a certain proportion to its quantity, but for want of an attractive force after it is saturated with water?' He goes on to put a great number of similar cases, all tending to the same point, that chemical combinations cannot be conceived in any other way than as an attraction of particles. [Note 9\6: Query 31.]