Analysis by Spectrum-observations. 95 



of which was about 60 cubic metres, we burnt a mixture of 3 

 milligrammes of chlorate of sodium with milk-sugar, whilst the 

 non-luminous colourless flame of the lamp was observed through 

 the slit of the telescope. Within a few minutes the flame, which 

 gradually became pale yellow, gave a distinct sodium line, which, 

 after lasting for ten minutes, entirely disappeared. From the 

 weight of sodium salt burnt, and the capacity of the room, 

 it is easy to calculate that in one part by weight of air there is 

 suspended less than 20 ~ 000 of a part of soda-smoke. As the 

 reaction can be observed with all possible comfort in one second, 

 and as in this time the quantity of air which is heated to 

 ignition by the flame is found, from the rate of issue and from 

 the composition of the gases of the flame, to be only about 50 

 cub. cent, or 0*0647 grm. of air, containing less than 200 o 0000 of 

 sodium salt, it follows that the eye is able to detect with the 

 greatest ease quantities of sodium salt less than 8 00 * 000 of a 

 milligramme in weight. With a reaction so delicate, it is easy 

 to understand why a sodium reaction is almost always noticed 

 in ignited atmospheric air. More than two-thirds of the earth's 

 surface is covered with a solution of chloride of sodium, fine 

 particles of which are continually being carried into the air by 

 the action of the waves. These particles of sea-water cast thus 

 into the atmosphere, evaporate, leaving almost iu conceivably 

 small residues, which, floating about, are almost always present 

 in the air, and are rendered evident to our eyesight in the 

 sunbeam. These minute particles perhaps serve to supply the 

 smaller organized bodies with the salts which larger animals 

 and plants obtain from the ground ; in another point of view, 

 however, the presence of this chloride of sodium in the air is of 

 interest. If, as is scarcely doubtful at the present time, the 

 explanation of the spread of contagious disease is to be sought 

 for in some peculiar contact-action, it is possible that the pre- 

 sence of so antiseptic a substance as chloride of sodium, even in 

 almost infinitely small quantities, may not be without influence 

 upon such occurrences in the atmosphere. By means of daily 

 and long-continued spectrum-observations, it would be easy to 

 discover whether the alteration of intensity in the line Na a 

 produced by the sodium in the air, have any connexion with the 

 appearance and direction of march of an endemic disease. 



The unexampled delicacy of the sodium reaction explains also 

 the well-observed fact, that all bodies after a lengthened exposure 

 to air show the sodium line when brought into a flame, and that 

 it is only possible in a few salts to get rid of the last traces of 

 the line Naa, even after repeated crystallization from water 

 which had only been in contact with platinum. A thin platinum 



