SCIENCE AND INDUSTRY. 
X-rays, obtained as explained above by filtering, is passed through the. 
specimen in the form of a fine powder. The effect of the regular arrange- 
ment of the atoms in crystals is to reflect this beam at certain definite angles 
characteristic of the crystalline substance and peculiar to it alone. These 
reflected beams along with the direct beam can be received upon a photo- — 
graphic film or plate and the angles of reflection thus determined. One — 
determination of the substance haying thus been made and recorded, its 
occurrence or non-occurrence in any sample of material can thus be ascer-_ 
tained by taking a single radiograph of the substance in question. Some 
characteristic spectra obtained by Hull in this way are shown in Fig. 10, 
Si 
Lif 
MgCo; 
MgO 
FIG.-10.—Typical X-ray Powder Patterns. 
“This method promises to be a valuable aid to ordinary chemical analysis 
in cases where it does not supplant it altogether. It has the great advantage’ 
over the latter method that the analysis is made on the material in its actual 
physical and chemical condition. Thus an analysis, of a finely-powdered 
mixture of sodium chloride and potassium nitrate by chemical means, which 
implies dissolving the material and breaking down the crystals, can give only 
the percentages of metals and negative radicals, not the manner of combina- 
tion. Hull’s method, however, reveals definitely the actual compounds 
existing in the mixture. It seems likely that in the metallurgy of alloys, 
~ jn ceramics, and other technical arts this method has an important future. 
538 
