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Rh must be invoked. This achieved, we may assert that we have arrived at that species of conceptual simplicity which we call a "law of nature." On a broad view, it is fair to say that, prior to Dalton, investigation and fancy pursued the one (i. e., the conception of "matter") through the many (i. e., these three aspects of the problem). For, on the whole, till we come to J. J. Becher (1635-82) and G. E. Stahl (1660-1734), the element theory held the field. And this is only to affirm that men were trying to master the properties of particular bodies, while reserving the remoter question of the ultimate constitution of "matter." Roger Bacon's view, probably the least fantastic we possess, is exceedingly significant of this.

The age of phlogiston, with its theories of combustion, marks a move to the second question. Men are now engaged in an effort to relate phenomena. Or, as Stahl puts it, in his conspectus: Combustible substance minus phlogiston is burnt substance—e. g., metals, sulphur, phosphorus, etc., minus phlogiston, are metal calxes, sulphuric and phosphoric acids, etc. On the other hand, burnt substance plus phlogiston is combustible substance—e. g., metal calxes, sulphuric and phosphoric acids, plus phlogiston supplied by carbon, are metals, sulphur, phosphorus, etc. In a word, the most different phenomena, such as the burning of carbon and the calcination of a metal, are shown to belong to the same class, and to be explicable by a simple conceptual hypothesis. Finally, when Lavoisier sent phlogiston by the board, the third question came to the fore, and men began to ask, How can we weigh, measure and enumerate the exact degree of relationship between the properties of substances? Dalton ranks among the great epoch-makers, because he first brought this inquiry within the range of practicable uniformity.

Discussions about prior discovery, over which much time and no little temper have been expended, prove profitless affairs, as a rule. You see, error and loyalty are human. For instance, I am well aware that scientific chemistry is dated usually from 1776, when Lavoisier made the balance the chemical instrument: but you will bear with Sadler and me if we travel a little farther back and, as loyal sons of alma mater, find the initial point in the classical investigation of latent heat, conducted by Black between 1759 and 1763, at Glasgow. Nevertheless, as Dalton's priority has been impugned, we are bound to consider the facts.