TRANSACTIONS OF SCIENTIFIC SECTION. 129 
cendental character and becomes a physical science. 
Its postulates ‘‘ are not necessary and universal truths, 
but merely axioms based on our experience of a certain 
limited region. Just as in any branch of physical in- 
—quiry we start by making experiments, and basing on our 
experiments a set of axioms, which form the foundation 
of an exact science, so in geometry our axioms are 
really, although less obviously, the result of expe- 
rience.” (Clifford, Common Sense of the Exact Sciences, 
Chapt. IV, § 19.) 
After a prolonged discussion of this interesting paper, 
LeRoy C. Cooley, Ph.D., read the following note rela- 
tive to the 
‘“ DECOMPOSITION OF THE ‘ELEMENT’ DIDYMIUM.’’ 
Dr. C. A. Von Welsback, has lately (June 18, 1885) 
sent a paper to the Vienna academy announcing the de- 
composition of the so-called element didymium. (See 
Chemical News, vol. 52, and Wature, vol. 32, p. 435.) 
He secured this result by treating the double nitrate 
of didymium and ammonium with a salt of lan- 
thanium. The two constituents differ by well marked 
characteristics. They have different atomic weights. 
The salts of one are green, of the other amethyst red. 
Each has a characteristic line spectrum and by mixing 
the two substances in proper proportions the product 
yields the spectrum of didymium. 
The decomposition of an ‘‘element”’ is in itself a re- 
sult of great importance ; the decomposition of an ele- 
ment by the ordinary chemical processes of the labora- 
tory isa result of still greater interest and importance, 
because, in addition to the fact of the compound nature 
of the element, it proves that new methods of research 
may not be needed in order to decompose them, and 
should incite chemists to continue their industrious use 
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