EVILS OF EXCESSIVE IMPURITIES IN IRON. 251 Whitney dwelt at considerable length upon the practicability of estimating the strength of iron or castings by analyses, and was of the conviction that the day was . not far distant when such would be generally accepted as being practical. How closely Mr. Whitney estimated the strength by analysis is shown by the relative estimated strength in Table 37. The general method of estimating the iron in cast metal is by deducting the total of the silicon, sulphur, manganese, phosphorus, and carbon percentages from 100.00. If there have been any errors in figuring these various percentages they would, by the above calculating process, be then thrown all on to the iron, so that as a check to positively determine the iron in metal it is really necessary to weigh up the iron after the other elements are taken away from it, when making the analyses, or make an analysis of the iron only and then let such be recorded in a column adjoining that of the totals for the carbons. Of course, wherever the " iron " is not shown in analyses it can, by the above plan, be estimated as far as such is to be valued and thus be made to serve for obtaining the 4' iron '' contained in any tests.ruary 5, 1901, wherein he stated that steel castings show only .074 to 1.44 per cent, of impurities and 98.56 to 99.86 per cent. iron.