PROTECTION OF IRON AND OTHER METAL WORK. 185 
turned to iron and its congener steel, which are to-day so 
freely used in all structural works. 
After the Bronze Age came that of the Iron, which must 
have covered a lengthy period of time, but with the dis- 
covery of the plentiful iron-ore deposits in the midlands of 
England, iron became greatly in demand for all structural 
purposes where its use could be economically adopted. The 
varieties of iron known to the engineer and builder were 
the wrought and cast, pig iron, white, grey, mottled, and 
malleable iron, and the metal associated with carbon, called 
steel. With the advent of the discoveries of Bessemer, | 
Lowthian-Bell, Siemens-Martin, and Thomas, the practical 
ironmaster became interested in the composition and 
chemical analysis of the metal, the presence of a per cent. 
or rather the fraction of a per cent. of carbon making all 
the difference in the world in the properties of the metal. 
At the present time an additional method of ascertaining 
the strength and fitness of quality of these metals has 
been introduced, namely the observation of the actual 
structure of the metal itself. By structure I mean of 
course microscopic structure. 
It has been found that the minute microscopic structure 
whereby the relative positions held by contiguous parti- 
cles can be seen is of greater significance than elemental 
composition as ascertained by means of chemical analysis, 
for within the last few years great advances have been 
made by graphological methods, a mode of examination 
effected by means of etching the polished metal with an 
acid. The highly polished surface of the metal to be tested 
is etched either by electrolytic methods or by immersion 
into free acids, halogens, hydroxides, or peroxides. This 
method was first applied to the sections or slices of meteor- 
ites, and shews that perfect continuity, or perfect homo- 
-geneousness is never found in matter as we know it: that 
M—Sept. 2, 1903. 
