This paper constitutes a progress report based on test data from 100 embrittlement detectors installed on operating boilers to ascertain the embrittling tendency of the respective boiler waters. Not until more than 10 years after the well-known sulphate-alkalinity ratios were formulated and published in the A.S.M.E. Boiler Code of 1926 were means devised whereby the tendency of an actual boiler water to cause cracking could be tested. Mention is made of the apparatus designed for laboratory use, developed by Straub and Bradbury, and the embrittlement detector of Schroeder and his associates for installation on an operating boiler. The tests reported by the authors were carried out by means of the latter. The procedure followed and the effects of various inhibitors used are given in some detail and the results are presented in the form of a series of graphs. The authors in conclusion suggest that boiler operators, faced with the problem of embrittlement cracking, take advantage of the testing methods now available, and determine the conditions which they can economically and effectively maintain in their boilers to minimize the difficulty.