The conclusion will depend on our conception of an atom and a molecule, and in the present state of our ignorance regarding these abstract entities no positive answer can be given. It appears certain that, on raising the temperature of argon, very little, if any, energy is absorbed in imparting vibrational motion to its molecules; and our choice lies between our ability or inability to conceive of a molecule so constituted as to be incapable of internal motion. If there be any truth underlying Professor Boltzmann’s conception, a molecule of argon cannot consist of any complex structure of atoms, otherwise it would possess more than three degrees of freedom, and heat would be utilised in causing rotational motions. As we know for a fact that the ratio between the specific heats of gases diminishes with the increasing complexity of their molecules, perhaps the safest conclusion is the one adopted by the discoverers of argon, that the balance of evidence drawn from _this_ source is in favour of its monatomic nature. But this hypothesis raises difficulties which are not lightly to be met. These difficulties arise from a consideration of the position of argon when it is classified with other elements. In 1863 Mr. John Newlands pointed out in a letter to the _Chemical News_ that if the elements be arranged in the order of their atomic weights in a tabular form, they fall naturally into such groups that elements similar to each other in chemical behaviour occur in the same columns. This idea was elaborated farther in 1869 by Professor Mendeléeff of St. Petersburg and by the late Professor Lothar Meyer, and the table may be made to assume the subjoined form (the atomic weights are given with only approximate accuracy):-- THE ELEMENTS ARRANGED IN THE PERIODIC SYSTEM. +---------------+---------------+----------------+--------------- | | | | | | | | |Lithium 7·0|Beryllium 9·1|Boron 11·0|Carbon 12·0 | | | | |Sodium 23·0|Magnesium 24·3|Aluminium 27·0|Silicon 28·3 | | | | |Potassium 39·1|Calcium 40·1|Scandium 44·1|Titanium 48·1 | | | | |Rubidium 85·5|Strontium 87·5|Yttrium 89·0|Zirconium 90·0 | | | | |Caesium 132·9|Barium 137·0|Lanthanum 142·3|Cerium 140·3 | | | | |? 170·0|? 172 0|Ytterbium 173·0|? 177·0 | | | | |? 221·0|? 225·0|? 230·0|Thorium 232·4 +---------------+---------------+----------------+---------------