But in extending the heating influence up to 210°, and in maintaining that temperature by means of a bath of paraffin for some hours before gradually reducing the same, he obtained a second modification of selenium, in which its conductivity increases with fall of temperature, and in which modification it is, therefore, analogous to the metals. This second modification of selenium is a better conductor of electricity than the first, and its sensitiveness to light is so great that its conductivity in sunlight is fifteen times greater than it is in the dark, as will be seen from the following table, in which is given the effects of different intensities of light on selenium (Modification II) obtained at Woolwich on the 14th of February, 1876:
—
| Selenium in | Relative Conductivities. | Resistance in Ohms. | |
| Deflections. | Ratio. | ||
| 1. Dark | 32 | 1·0 | 10,070,000 |
| 2. Diffused daylight. | 110 | 3·4 | 2,930,000 |
| 3. Lamplight | 180 | 5·6 | 1,790,000 |
| 4. Sunlight | 470 | 14·7 | 680,000 |
Unfortunately, however, the second modification is not so stable as the first; when lowered in temperature parts of it change back into the first or metalloid modification by taking up specific heat, and in watching this effect a point is discovered at which ratio of increase of conductivity with fall of temperature changes sign, or where the electrolyte substance appears to predominate over the metallic selenium. If cooled down to 15° C., the whole of the metallic selenium is gradually being converted back into the first variety. The physical conclusions here arrived at may be said to be an extension of Helmholtz’s theory that the conductivity of metals varies inversely as the total heat contained in them. Helmholtz had only the sensible heat of temperature (counting from the absolute zero point) in view, but it has already been shown by Hittorf and Werner Siemens that it applies in the case of tin and some other metals, also to specific heat and to the latent heat of fusion. In selenium the specific heat is an extremely variable quantity, changing in the solid mass at certain temperatures, and, it is contended, under the influence of light. Aided by these experimental researches, my brother arrives at the conclusion that the influence of light upon selenium may be explained by a “change of its molecular condition near the surface, from the first or electrolyte into the second or metallic modification, or in other words, by a liberation of specific heat upon the illuminated surface of crystalline selenium, which liberated heat is reabsorbed when the liberating cause has ceased to act.” Professor Adams, who has likewise investigated this singular action of light upon selenium, ascribes it to a different cause. He says:—
1. That the light falling on the selenium causes an electromotive force in it in the same direction as the battery current passing through it, the effect being similar to the effect due to polarisation in an electrolyte, but in the opposite direction.
2. That the light falling on the selenium causes a change on its surface akin to the change which it produces on the surface of a phosphorescent body, and that in consequence of this change the electro-current is enabled to pass more readily over the surface of the selenium.
SEM′OLA (Bullock’s). This preparation consists of wheaten flour deprived of much of its starch by washings with water, and contains the largest amount (48 per cent.) of nitrogenous or albumenoid principles consistent with its adaptability to culinary purposes. It is specially intended as a food for infants, weakly children, and invalids.
SEMOLI′NA. Syn. Sémoule, Semoulina. The large hard grains of wheat flour retained in the bolting machine, after the fine flour has passed through its meshes. “The best sémoule is obtained from the wheat of the southern parts of Europe. With the sémoule the fine white Parisian bread called ‘gruau’ is baked.” (Ure.)
SEN′EGA. Syn. Seneka, Snakeroot, Rattlesnake r.; Senegæ radix (B. P.); Senega (Ph. L., E., & D.), Radix senegæ, L. “The root of the Polygala Senega, Linn.” (Ph. L.) A stimulating diaphoretic, and expectorant; in large doses diuretic, cathartic, and emetic. In America it is used as an antidote to the bite of the rattlesnake. Drs Chapman and Hartshorne extol it as an emmenagogue. Dr Pereira says that it is an exceedingly valuable remedy in the latter stages of bronchial or pulmonary inflammation, when this disease occurs in aged, debilitated, or torpid constitutions.—Dose, 10 to 30 gr., in powder or decoction (combined with aromatics, opium, or camphor), thrice daily.
According to Patrouillard senega is occasionally adulterated with the roots of Asclepias vincetoxicum. The branches of the latter root are cylindrical, very white, and almost devoid of taste; those of senega, on the contrary, are yellowish and twisted, and have a very acrid taste. The froth produced by shaking an infusion of senega keeps much longer than that produced by an infusion of the adulterant. In other respects there is a great resemblance between the two roots.