magnesium, "light calcined magnesia," on the other hand, was found. to bear the heating test well, losing at first a small amount of weight, in drying, probably, but remaining constant in weight at high temperature thereafter.

Accordingly we undertook to repack our Thomson effect apparatus, using now the dried magnesium oxide, and to repeat the experiments on lateral flow through the packing as well as those on the temperature coefficient of the thermal conductivity of iron.

As we had a strong apprehension that our boilers would now leak vapor of water or of naphthalin into the packing space, though we had done what we could to stop such leaks as we had discovered, and as we feared that any condensation of vapor in the powdery oxide would be highly objectionable, we put a sort of buffer of asbestos fibre about 0.5 cm. thick against the boiler at each end of the space to be packed, trusting that the upward movement of air through this layer would carry off vapor coming into it. Then, after putting in place the wrapping of magnesia and asbestos which lies outside the outer guard-ring (see Figure 3), we poured the magnesium oxide down into the space between the many bars, striving, by tapping the outer covering and by pressing down the oxide with strips of metal, to get the light powder so well packed that it would not by subsequent settling leave empty spaces under the bars. The density of the powder as thus placed was about 0.17 gram per cubic centimeter.

After this we made, in spite of a considerable amount of leakage of naphthalin, especially toward the last, observations which resulted as follows:

Lateral outflow from each main bar per 1° difference of temperature between the main bars and the inner guard-ring,

0.00791 cal. with both pots at 100°, July 17-18

Temperature coefficient of thermal conductivity in iron between 115° and 204°, with reference to the value at 115°,

f0.00076, July 19 and 24.

It is to be noted that in this packing the thermal conductivity appears to be about 30 per cent greater at 218° than at 100°.

VOL. XLII.-40

No great confidence should be placed in the numerical accuracy of this indication, but the direction of the change seems to be well established.

The cause of the apparent increase in conductivity of the packing, both at 100° and at 218°, with lapse of time is not known.

The value here found for f, combined with that found for the same coefficient when asbestos packing was used, gives as a mean

f(0.00076 +0.00060)=0.00068.

Proceedings of the American Academy of Arts and Sciences.

VOL. XLII. No. 23.- MARCH, 1907.

CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOÖLOGY AT HARVARD COLLEGE. E. L. MARK, DIRECTOR. - No. 187.

AN ELECTRIC WAX-CUTTER FOR USE IN
RECONSTRUCTIONS.

By E. L. MARK.

CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOÖLOGY AT HARVARD COLLEGE. E. L. MARK, DIRECTOR. - No. 187.

AN ELECTRIC WAX-CUTTER FOR USE IN
RECONSTRUCTIONS.

BY E. L. MARK.

Presented December 12, 1906. Received December 17, 1906.

ANY ONE who has had much experience in making models of microscopic objects by means of wax reconstruction-plates must have felt the disadvantages of the ordinary method of cutting out the plates with a scalpel.

Some years ago the advantages of heating the scalpel in a flame suggested to me the desirability of employing a knife heated to a constant temperature, and observation of the methods employed in making corrections to "line process" plates prepared by the so-called wax method led me to think it might be possible to use for this purpose a small gas jet attached to the knife and connected with the gas supply by a small flexible rubber tube. I did not succeed, however, in producing a device that would work satisfactorily. A little later it occurred to me that a wire heated by an electric current might be kept at a sufficiently constant temperature to answer the purpose. Accordingly in an ordinary bow-saw with large bow the saw blade was replaced by a fine wire, the ends of which were insulated from the frame and connected by means of flexible insulating wire with a 110-volt alternating electric circuit. By introducing into the circuit the proper resistance in the form of electric lamps arranged in multiple-it was possible to heat to the proper degree the wire selected. By means of this apparatus one could melt a wax plate readily along any predetermined course, provided the wire were slowly moved back and forth as in sawing. But this apparatus was defective, owing to the lengthening of the wire and its consequent laxness when heated. It became obvious at once that for accurate work some sort of spring would have to be introduced into the mechanism to take up the slack of