istically chosen. The psychological principle of similarity, which so occupied my mind from first to last, had recently been followed out on its emotional side,-namely, the throb or flash of pleasure attendant on all great strokes of similarity in diversity. I dwelt upon this topic in the introductory lecture, which admitted of abundance of illustration in the physical sciences.

As already explained, I had suggested to the editor of the Westminster a scientific topic for a second article. On the 20th of December, he accepted my proposal for a paper of sixteen pages on "The Accurate Investigation of the Properties of Matter," to appear in April or in July. This, of course, became part of my occupation in the early months of 1841.

This essay rose above the region of mere exposition to the ambitious attempt of laying down rules of inductive discovery or invention, by the help of a a definite procedure. The procedure was divided into four distinct stages or prescriptions-namely, "to express every fact in as definite language or by as definite and true parallels or imagery as it is possible to use; to express it, also, in as many different ways as possible, provided each is precise; to give separate expression and attention to every part into, which a fact can be divided; and to extract

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from every fact all the conclusions that are necessarily bound up with it". The rules were copiously exemplified in connexion with the atomic constitution of matter, which was regarded as made up of ultimate particles under the balance of the opposing forces of attraction and repulsion. The situation was viewed in every possible aspect, from the compact condition of solid matter to the altered condition of the gaseous state, where one force alone-repulsion -appeared to be present. The example that possessed most interest as the upshot of the research was the suspension of steam particles in the air when their repulsive force was withdrawn under cooling. It was inferred, with apparently irresistible cogency, that particles of visible vapour or cloud were kept from descending to the earth under gravity by their adhesive attraction to particles of air, and that their formation into raindrops was necessarily a protracted operation. This last inference has been confirmed in a startling way by Tyndall's discovery that, but for the intervention of dust particles, the formation of raindrops would be quite impracticable.

The reception of the article in the Westminster was, on the whole, very favourable. Mr. Grote wrote to the editor, expressing his opinion of its merits, and desiring to know if the author had

written any other papers on Natural Philosophy. He also spoke to Mill in the same strain. This may be said to have been the beginning of my acquaintance with Grote, although it was several years before we became personally acquainted. Professor Spalding of St. Andrews perused the article, in connexion with my application for the Natural Philosophy Chair in 1846; and it helped to determine him to support my claims. The article did not find the same favour with the authorities on physical science with whom I came into contact. I knew Clark too well to expect him to allow any merit to the mode of procedure recommended, and I never brought the article under his view. It was somewhat different with Graham, who got to know what I was driving at through some observations I made on a paper of his at the British Association in 1845. His remark was that the processes inculcated would very soon reach their limit-which was quite true, but did not necessarily render them useless, as the examples actually showed. Both Graham and Clark had a supreme faith in experiment as such, of which they were preeminently masters. Neither put much stress upon mathematics; not being mathematicians themselves, and not dealing in the class of researches most suitable for its aid. Still less

would they think that any discoveries could be made by mere separation of complex facts into their several constituents, with the view of concentrating attention upon one at a time. As to laying down a rule for formally drawing up a list of examples under a given generality, they and most other scientific inquirers would consider the direction superfluous. Notwithstanding all this discouragement, I entertained for the time the idea of inculcating an art of discovery, as Bacon had done in the Norum Organum. His directions were very generally considered futile ; still, something was done even by Herschel and by Whewell in their generation to follow in his wake, with improvements in the detail. It was my belief that pupils might be so indoctrinated as to prepare them for the intellectual side of research; yet, in point of fact, I never did succeed in making the requisite impressions upon any one but myself. During the whole of my studious career, I had in view those maxims of procedure, although I seldom-yet sometimes-went through all the formalities embodied in the article in question. In the Inductive Logic, under the heading, "Art of Discovery," I brought together every device within the compass of my knowledge or experience that could claim to be helpful to the mind in original investigation, whatever the subject might be.

Another composition falling within the same session, was an essay on a psychological topic. entitled "On the Use to be made of a Precise Knowledge of the Limits of the Human Faculties". The stress of the paper was laid upon the fact that, broadly speaking, the human mind can attend to only one thing at a time; in which respect a contrast is made between humanity and divinity. The practical carrying out of the supposed limitation is to analyze all complexes into their smallest parts, and to concentrate the mind upon each seriatim. In point of fact, the essay was another exemplification of the process which was uppermost in my early published papers,-viz., of exhaustive study of each individual fact, or part of a fact, that could, for the moment, be taken in isolation. In all probability, the composition of the essay went on in conjunction with the paper for the Westminster.

The remaining incident of importance in this eventful session was a visit to Edinburgh, early in March. My friend Robertson, having interested himself very much in the Non-Intrusion controversy as a political question of the day, had visited Edinburgh to converse with the leaders. He took the opportunity to introduce my name as one that might be made available for some