INDUSTRIAL AND ENGINEERING CHEMISTRY
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completely that, when the former is well distributed in rubber, a section from the microtome appears jet black when viewed by transmitted light. Such a section is, of course, quite useless for studying the structure of the stock by means of the microscope. Sections of any desired thickness may be made by the process described in the following paragraphs. This method is, in short, a modification of that described by Greens4 Pieces of technically cured stock are given a treatment with a solution of sulfur chloride in carbon tetrachloride. As Green has already pointed out, the treatment with sulfur chloride depends upon the type of the stock in question and also upon its previous history as to cure. The writers have found by experience that a technically cured stock containing 19 per cent of carbon black by weight gives the best results when treated for one and one-half hours a t room temperature in a solution which consisted of four drops of sulfur chloride in 10 cc. of carbon tetrachloride. The treatment with curing acid causes the rubber to swell c o n s i d e r a b 1y , b u t , what is much more important for subsequent steps of the process, the stock is rendered “dead.” Most of the original tenacity has disappeared and the stock has little tendency to regain its shape after it has been released from stress. The percentage stretch a t rupture is very low and thin s e c t i o n s may be easily made with the microtome Figure 4 b e c a u s e t h e stock cuts rather than squeezes away from the edge of the knife. Moreover, the operator may decrease the thickness of the microtome sections on a microscopic slide to any desired degree by pressure skilfully applied with a blunt instrument. Herein lies the secret of making thin sections of carbon black stocks. The mass must be brought to such a state by the sulfur chloride treatment that the microtome sections mill not recover their former shape after they have been squeezed thin. They must adhere to glass, something that raw or technically cured rubber will not do. If the stock is allowed to stand for several hours after the sulfur chloride treatment, it will contract and become brittle. Brittle sections cannot be thinned out by pressure, because the latter produces fragments or a powder instead of a broad, flat surface. It is often possible to soften a brittle section by soaking it in carbon tetrachloride. It has also been found expedient to wet the specimen with carbon tetrachloride while the squeezing operation is being carried out. A freshly prepared wet specimen can be smeared over the surface of the microscopic slide like a portion of very resistant putty. I t is not a t all necessary, however, to employ such extreme measures. After removal from the sulfur chloride-carbon tetrachloride solution the specimens are shaped as represented by A , Figure 4. Convenient dimensions are, height 10 mm., thickness 0.5 mm., width at base 4 mm. The object of the wedge shape is to present a small portion of the specimen to the cutting edge of the sectioning razor and a t the same time t o insure a rigid mounting in the paraffin block, a process described below. The wedge-shaped specimen is next mounted in a paraffin block with the apex of the wedge up. The shape of the paraf-
B A
B
4
TFUSJOURSAL. 13, 1130 (1920).
Vol. 17, No. 9
fin block r d l be determined, of course, by the style of microtome employed. For purposes such as those under discussion a most excellent microtome can be made by utilizing ordinary microscopic equipment as fully described in another article.6 The paraffin block used in the writers’ experiments was shaped much like a microscopic objective. B, Figure 4, is a sketch of such a block. showing the apex, C, of the rubber specimen set even with the rectangular end of the block. Preparation of Slide
The thinnest section that it is possible to make with the microtome is transferred to the center of a microscopic slide. With the aid of a magnifying glass, held in place by a suitable stand, the specimen is next pressed out thin on the glass slide by means of a blunt instrument, such as the small blade of a pocket knife, the point of which is not sharp. The pressure is best applied so that the hand travels through a short arc of a circle. The specimen and the point of the knife blade remain in the center of the imaginary circle, whereas the blade and handle of the knife form a radius. Simultaneously with the to-and-fro movement the knife is withdrawn slightly in the direction of the hand. The thinning operation must be continued until the specimen has the color of the undertone of the carbon-reddish amber or blue-gray, as the case may be, when the slide is held between the eye and daylight. It is usually necessary to observe the color through a magnifying glass. It is quite useless to mount a specimen that appears jet black under the conditions enumerated above, because the black color is proof that light will not pass from the microscopic reflector through the specimen into the objective. Skill on the part of the operator is, of course, essential to success, but the most important factor is the condition of the stock during the thinning process by pressure. If the stock is too hard it will break into fragments each of which is impermeable to light. On the other hand, if the treatment with sulfur chloride has not been carried far enough the specimen will be elastic and will not remain spread out in a thin film after the pressure is removed. It is wise to keep the glass and pressure instrument wet with carbon tetrachloride during the operation, as this precaution enables the stock to adhere to the glass slide. The slide is then mounted with Canada balsam in the usual manner, A very thin cover glass should be employed and should be pressed down between two pieces of glass or microscopic slides while the balsam is quite warm. This is necessary because the specimen is pressed on the surface of the slide and cannot be brought into the focus of a high-power objective if the cover glass and the Canada balsam are too thick. 6
See page 894. this issue.
Simplified Color Photography Revealed to French Academy A new method of color photography was recently revealed by J. L. Breton, a t a meeting of the Academy of Sciences in Paris. It is reported, however, to be the result of an invention and experiments by G. Rousseau in this field. Instead of the old method, requiring three separate films in the cameras, provided with red, blue, and yellow screens and conducting reverse operations in developing a single plate, M. Rousseau discovered that he could get better results by a single exposure of three superimposed films and the development of a single plate. By placing three films one upon the other in the camera, 11. Rousseau found that the first film registered blue and violet; the second green, while yellow, orange, and red went through the first two films and registered on the third. The advantages of the new process are that the films give absolutely identical views and the time exposure has been reduced to-one twenty-fifth of a second-much faster than with the color screens and three separate exposures.
