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however, placed in an amalgamated vessel after a short time becomes 

 covered with a singular film of amalgam, which impairs the reflect- 

 ing power of the surface, and if at all agitated, soon entirely destroys 

 it. And this is the case even when the vessel is made of amalga- 

 mated platina. The most important, and by far the most difficult 

 part of Mr. Pritchard's experiments, consisted in the invention of a 

 method by which these films can be easily and practically removed. 

 The details, many of which are curious and interesting, would here 

 occupy too much space, but they are fully explained in a memoir 

 recently read to the Royal Astronomical Society of London ; and it 

 may be added, that the process has been adopted at the Royal Obser- 

 vatory at Greenwich, and is now in progress of trial at the Observa- 

 tories of Paris and Cambridge. 



April 25, 1853. 



Professor Challis gave a lecture on the Adjustments of a Transit 

 Instrument. 



A paper was read by Professor De Morgan on the Principle of 

 Mean Values, and an addition to a paper on the Symbols of Logic, 

 &c. in vol. ix. part 1 of the Society's Transactions. 



Though the heading of this paper describes one of its main results, 

 yet it might with equal propriety have been styled a discussion of 

 some points of algebra, with reference to the distinction of form and 

 matter. This distinction, it is contended, is more extensively applied 

 in algebra than in logic, though more recognized in logic than in 

 algebra. Looking at the disputed points which exist in the higher 

 parts of mathematics, and feeling satisfied that they will never be 

 settled until the separation of form and matter is both visible and 

 complete, the author makes a first attempt towards the examination 

 of the question how far this yet remains to be done. A number of 

 comparisons are made between algebraical and logical process, in 

 the course of the inquiry, illustrative of the opinion entertained by 

 Mr. De Morgan, that logic, as treated, requires the interposition of 

 the algebraist, and cannot, except by aid of algebraical habits, be 

 rendered a complete exposition of the forms of thought. In digress- 

 ive notes, he combats the opinion that a generalization of the quan- 

 tity is, as asserted, a new material introduction. He argues against the 

 too mathematical tendency of some of the logicians who have endea- 

 voured to extend the ancient system, especially the attempt of some to 

 make the logical import of the proposition nothing but a comparison 

 of more and less, and an equation or non- equation of quantities. He 

 points out that the proposition has been formalised in nothing but 

 its terms, subject and predicate ; and gives an instance of the method 

 in which a failure of general maxims is answered by the sole asser- 

 tion that the mode of expression which brings about the failure is 

 useless. He refers to what were called sophisms, contrasting the 

 neglect of them by the logicians with the use which the algebraists 

 have made of their corresponding difficulties, as in the case of nega- 

 tive and imaginary quantities, the fraction £, &c. He argues against 

 the assertion of more than one eminent writer on logic, that the 



