102 Prof. W. X. Hartley and Mr. H. Eamage. The Mineral 



Le Hope-o-Torm. 



Plate 312. 



Spectrum 1. — The ammonia precipitate — 



Similar to Nos. 1 and 4. 



„ 2. — The oxalates — 



Similar to Nos. 2 and 5, but the silver was not so 

 strong. 



3. — Magnesia and the alkalies — 

 Similar to Nos. 3 and 6. 



It is necessary to explain that the symbol for magnesium and the 

 alkaline earth metals refers generally to the oxides. With magnesium, 

 in fact, this is always so, since the bands of the oxide magnesia alone 

 are visible. In the case of calcium, the blue line 4226 is photographed 

 when only a small quantity is present, but the bands of calcium oxide 

 are the chief feature of the spectrum when the base is in larger propor- 

 tion. Where the symbol is printed in italics it indicates a trace of the 

 substance, and where followed by a note of interrogation it is not quite 

 certain if even a trace is present, as, for instance, where only one of two 

 rubidium lines is seen, there being two iron lines occupying almost 

 the same positions ; or where one of the gallium lines is barely visible, 

 and the second is enveloped by manganese lines. The relative strength 

 of the lines, as seen by comparing the different spectra, is, in some 

 instances, indicated on the tabulated statement by suffixes, the num- 

 ber 1 indicating the weakest line and 10 the strongest. 



The difference in the number of the iron lines is a measure of the 

 quantity of iron present as metal or otherwise, and a comparison of the 

 strength of the lines also indicates the relative quantity of substances. 

 The results in many cases are quantitative, inasmuch as the same weight 

 of material was taken. 



On the Nature of Dust from the Clouds. 



The principal characteristic of dust which has fallen directly from 

 the clouds or collected by hail, snow, sleet, or rain, is its regularity in 

 composition — each specimen appears to contain the same proportions of 

 iron, nickel, calcium, copper, potassium, and sodium. The proportion 

 of carbonaceous matter must be small, otherwise a diminution in the 

 proportion of the metals present would render the metallic lines 

 weaker. There is a very considerable difference between the dust from 

 sleet, snow, and hail suddenly precipitated, the difference being in the 

 proportion of lead, which, in the dust from sleet, is much larger than 

 in the other specimens, though dust from hail and .one quantity 

 collected from rain contain more than is found in any other specimens 



