INERT CONSTITUENTS OF THE ATMOSPHERE. 589 



that gas is somewhat soluble in water (4.1 volumes per 100 of water 

 at 15° C.) ; but some also contain helium, as for instance the gas from 

 the Bath springs, which Lord Eayleigh foimd to contain argon mixed 

 with about 8 per cent, of its volume of helium; and helium has also 

 been found in mineral springs at Wildbad, and at Cauterets, in the 

 Pyrenees. It would appear, then, that helium is not such a very rare 

 constituent of our globe; and indeed, it is probable that it is contin- 

 ually escaping from the earth in small quantities in certain regions. 



Let us next turn our attention to the atomic weights of these ele- 

 ments, in order to discover what position should be assigned to them 

 in the periodic table. It is not difficult to ascertain their molecular 

 weights; that is the relative weights of equal numbers of molecules; 

 for, assuming Avogadro's hypothesis, that equal volumes of gases 

 contain equal numbers of molecules, or particles capable of independent 

 existence in space, the weights of equal volumes of these gases, com- 

 pared with that of an equal volume of oxygen taken as 16 (the usual 

 standard) will give the relative weights of their molecules. The 

 density of helium was found to be very nearly 2, or one eighth of that 

 of oxygen; while that of argon was 19.94, very nearly 20. It may be 

 interesting to spend a few minutes in a description of the method by 

 which the density is determined. The principle is to weigh a gtobe, 

 completely emptied of air by means of an air-pump ; the globe is then 

 filled with the gas, care being taken to observe accurately the temper- 

 ature and pressure of the atmosphere at the moment of closing the 

 globe ; and the difference in weight of the full and the empty bulb gives 

 the weight of a known volume of the gas. It is easy to compare it 

 with that of an equal volume of oxygen. 



The weight of a gas is much more considerable than might be sup- 

 posed. Thus, a liter of oxygen weighs nearly a gram and a half; and 

 the air in an ordinary room twelve feet broad, long and high, weighs 

 over a hundred and fifty pounds. It is possible to obtain fair results 

 by weighing as little as 30 cubic centimeters, or about one fluid ounce 

 of any gas. Such a globe filled with helium weighs about one two- 

 hundredths of a gram ; and it is not difficult to be fairly certain to one- 

 hundredth part of that weight. With a heavier gas like argon, much 

 greater accuracy is of course possible. 



Now, although the standard atomic weight with which others are 

 compared, that of oxygen, is taken as 16, it is believed, for reasons 

 which will afterwards appear, that a molecule of oxygen consists of two 

 atoms, the weight of which will of course be 32. And, as the weight 

 of helium is one eighth of that of an equal volume of oxygen, the 

 weight of a molecule of helium will be the eighth part of 32, or 4. 

 Similarly, the molecular weight of argon compared with that of an 

 atom of oxygen taken as 16 will be 40, But the question has stiU to 



