On the other hand, the diagram of equilibrium curves could have been constructed independently of micrography-e.g., by the thermal method. In fact, it has been by the co-operation of the two methods that they have been controlled and, in certain circumstances, completed.

Practical Conclusions.-Setting aside all theoretical speculation, we have seen that—

1. The temperature of heating,

2. The temperature of quenching,
3. The rapidity of cooling,

that is to say, the principal circumstances of the calorific treatment of steels, reveal themselves in the variations of structure, with a precision which the mere inspection of fractures is certainly far from furnishing. But to get any industrial results from these facts, it would be necessary to have the different aspects of the structure correlated with the corresponding mechanical properties.

The practical application of metallography to Metallurgists and Engineers requires, then, for each interesting metal, a preparatory study. But, when this study is made, it will be easy to determine with sufficient exactness the calorific treatment undergone by a finished article or piece of metal, to see if this treatment, which is of first importance, has conformed or not to given specified rules, to rectify it if needful, and to take the responsibility in case of rejection. Mr Albert Sauveur, who edits that excellent special review The Metal

lographist, had already, when engineer to the Illinois Steel Company, organised a regular system of micrographic analysis.1

For industrial applications the complete method which has been described in this paper will not, as a rule, be necessary. Like the general rules of chemical analysis, this method allows, according to circumstances, simplifications suggested by experience. It is a method of investigation which may be laborious in the case of original research, but which will, eventually, be much simplified.

1 Trans. Amer. Soc. of Min. Eng., t. xxii. p. 546.

APPENDIX I.

APPARATUS EMPLOYED FOR MICROPHOTOGRAPHY.

THE whole of the appliances figured are by Nachet & Fils, of Paris, and they give complete satisfaction (fig. K).

The microscope is a present-day model for the examination of transparent substances.

A total reflection prism is fixed above the objective to illuminate opaque bodies. That arrangement, however, has the inconvenience of reducing the utility of the microscope, and microscopes such as are made by Nachet and others at the present time are certainly to be preferred.

The microscope stands on a table of suitable height, under a movable verticle camera which slides between two uprights. I consider the verticle to be very much superior to the horizontal camera for studying metals. The chief reason for this is, that pieces of varying shape, seldom terminated by two parallel planes, have to be examined. In that case the mounting of the specimen is done very quickly and correctly by putting a little soft wax (sculptors' modelling cement) upon a glass plate and embedding the specimen in the wax. The

levelling is managed by means of two pieces of glass

tube of equal height placed one on each side of the specimen.

A sample so mounted would often move if it were placed vertically, and it would be necessary then to replace the wax by a cement which would harden, or by cutting the metal so that it has two parallel planes. Besides, with the vertical camera the microscope is in the best position for use, and when an interesting point is noticed and the photograph of the specimen required, the camera is simply let down and joined to the fixed socket of the microscope.

The micrometer focussing screw has a forked aluminium lever, and this is worked with a rod.

The space between the uprights gives freedom for the movement of the camera. The only inconvenience, which is, however, not serious, is that it is necessary to stand upon a stool to bring the camera into focus for photographing.

Nachet's No. 2 eye-piece is generally used, and, when measurements are required, a micrometer eyepiece.

For photographing, the ordinary eye-piece is replaced by one for projection. According as the draw-tube is extended or shortened, and as the projection eye-piece itself is more or less extended, the magnification is modified, the extension of the camera being constant.

To obtain the least magnification with each objective, the projection eye-piece is removed and a velvet tube. introduced into the draw-tube to avoid reflection upon the photographic plate.

The set of objectives comprises Nachet's Nos. 0, 2, 3, 5, 7, and 9.