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THE PRESIDENTIAL ADDRESS. 17 
The element chlorine has had its atomic weight repeatedly deter- 
mined, and, for special reasons, with the highest attainable accuracy. 
On the oxygen standard it is 35.46, and this value is accurate to the 
second decimal place. All attempts to prove that it is a whole 
number—35 or 36—have failed. When, however, the gas is analysed 
by the same method as that used in the case of neon it is found to 
consist of at least two isotopes of relative mass 35 and 37. There is no 
evidence whatever of an individual substance having the atomic weight 
35.46. Hence chlorine is to be regarded as a complex element con- 
sisting of two principal isotopes of atomic weights 35 and 37 present 
in such proportion as to afford the mean mass 35.46. The atomic 
weight of chlorine has been so frequently determined by various 
observers and by various methods with practically identical results that 
it seems difficult to believe that it consists of isotopes present in definite 
and invariable proportion. Mr. Aston meets this objection by pointing 
out that all the accurate determinations have been made with chlorine 
derived originally from the same source, the sea, which has been perfectly 
mixed for «eons. If samples of the element could be obtained from 
some other original source it is possible that other values of atomic 
weight would be obtained, exactly as in the case of lead in which the 
existence of isotopes in the metal found in various radioactive minerals 
was first conclusively established. 
Argon, which has an atomic weight of 39.88, was found to consist 
mainly of an isotope having an atomic weight of 40, associated to the 
extent of about 3 per cent., with an isotope of atomic weight 36. 
Krypton and xenon are far more complex. The former would appear 
to consist of six isotopes, 78, 80, 82, 83, 84, 86; the latter of five 
isotopes, 129, 131, 132, 134, 136. 
Fluorine is a simple element of atomic weight 19. Bromine con- 
sists of equal quantities of two isotopes, 79 and 81. Iodine, on the 
contrary, would appear to be a simple element of atomic weight 127. 
The case of tellurium is of special interest in view of its periodic relation 
to iodine, but the results of its examination up to the present are 
indefinite. 
Boron and silicon are complex elements, each consisting of two 
isotopes, 10 and 11, and 28 and 29, respectively. 
Sulphur, phosphorus, and arsenic are apparently simple elements. 
Their accepted atomic weights are practically integers. 
All this work is so recent that there has been little opportunity, 
as yet, of extending it to any considerable number of the metallic 
elements. These, as will be obvious from the nature of the methods 
employed, present special difficulties. It is, however, highly probable 
that mercury is a mixed element consisting of many isotopes. These 
have been partially separated by Bronsted and Hervesy by fractional 
