92 



SCIENCE. 



[Vol. XV I II. No. 445 



spectrum of a solution with concentration, the above experiment 

 has an obvious defect, viz., that the thickness of the layer of the 

 strong and weak solutions being equal, the nutnbers of the salt 

 molecules through which any ray of light passes are very differ- 

 ent in the two cases. It should therefore be supplemented by 

 showing also the color or the spectrum obtained when the light is 

 passed through a wide trough of the dilute solution, the ratio of 

 the widths of the troughs being the reciprocal of the ratio of the 

 percentages of salt in the two solutions. 



(3) Dr. W. W. J. Nichol's observation (Phil. Mag., Ser. 5, xix., 

 453) that anhydrous sodium sulphate will dissolve in a supersatu- 

 rated solution of that salt may readily be shown as a lecture ex- 

 periment by projection. For that purpose place a test tube con- 

 taining the solution in a trough with glass sides full of water, and 

 focus it on the screen. Then let the anhydrous salt in the Jorm 

 of a fine powder, fall upon the surface of the solution. By taking 

 a pinch of the powder between the thumb and forefinger (both 

 being quite dry), it may be made to fall as a shower of fine parti- 

 cles. These pass into the solution and are seen to move slowly 

 across the screen through the solution, dissolving as they go, in 

 some cases disappearing, and often changing the concentration of 

 the part of the solution through which they have passed, so as to 

 produce obvious refraction efEects. Finally, to show that the so- 

 lution was supersaturated, add a few crystals of the hydrated 

 salt and crystallization at once occurs. The anhydrous salt must 

 be added as a shower of fine powder, as larger pieces may — by 

 taking up water and forming crystals of the hydrated salt before 

 they can dissolve it — give rise to a general crystallization of the 

 solution. 



(4) The peculiarity of the solubility in water of such substances 

 as aniline, carbolic acid, etc., observed by Alexejew (Wied. Ann. 

 Bd. XXVIII., 305), may readily be shown on the screen, by using 

 cai-bolic acid, -^hose critical temperature (the temperature above 

 which it and water are mutually infinitely soluble) is about 69° 

 C. For this purpose, pour some of the acid into a long test-tube, 

 of about twelve or fifteen millimetres in diameter, and add water. 

 The water will lie in a layer above the acid. Support the test- 

 tube by a clip grasping at the top, and focus on the screen. The 

 line of demarcation between the two liquids will be evident. 

 Now mix the liquids by stirring, and the whole becomes cloudy. 

 Let the tube stand, and the liquid separates again into two layers, 

 having difi:erent depths from those they had before, both being 

 now solutions. As this process requires considerable time, the 

 stirring may have been done beforehand. Next surround the 

 test-tube by a beaker of boiling water, passing it upwards from 

 below, and stir the liquids with a hot glass rod. A slight cloudi- 

 ness appears, but the liquid quickly clears and is seen to have be- 

 come homogeneous throughout, the line of demarcation having 

 disappeared. If now the beaker of hot water be removed, and 

 one of cold water be substituted for it, the liquid becomes cloudy, 

 a strong solution separating out everywhere, and the little spheri- 

 cal masses of strong solution sinking and coalescing as they sink, 

 to form larger spheres. After a time the liquid is seen to have 

 again become separated into two layers. If the necessary time is 

 not available, the separation into layers may be obtained very 

 quickly by removing the beaker of cold water and again applying 

 the hot bath, which, raising the temperature, stops the separating 

 out of the strong solution and re-dissolves it in the surrounding 

 weaker solution, thus producing a comparatively strong solution 

 in the lower part of the tube and a comparatively weak one in 

 the upper part. The experiment requires but a few minutes and 

 is both striking and instructive. 



SOME DISEASES OF LETTUCE AND SUCUMBERS. 



DuEiNG the past winter and spring James Ellis Humphrey, 

 professor of vegetable physiology at the Massachusetts Agricultu- 

 ral Experiment Station, has been engaged in the study of certain 

 diseases of lettuce and of cucumbers, cultivated under glass. 

 The investigation of some of these is sufficiently advanced to jus- 

 tify the following preliminary announcement, given in Bulletin 

 No. 40 of the station mentioned. 



The rotting of lettuce has been a source of much loss to gar- 



deners who cultivate that plant as a winter crop, but its cause, 

 and, therefore, proper preventive measures, have not been known. 

 It usually appears first just above the surface of the soil at the 

 attachment of the lower leaves to the stem, and then spreads to 

 the centre of the head, causing the stem and the bases of the 

 lower leaves, and later the whole of the tender inner leaves, to 

 become decomposed into a slimy mass. The larger leaves being 

 thus cut off from the stem by decay at their bases usually dry up ; 

 and there appears after a time, on the remains of the plant, if 

 left undisturbed, the erect, spore-bearing threads of one of the 

 imperfect fungus forms known by the name Botrytis or Polyaatis. 

 The vegetative threads of this fungus are to be found in the de- 

 caying tissues of the host in the early stages of the trouble, and no 

 other fungus has ever been observed in connection with it. The 

 professor's observations make it practically certain that the dis- 

 ease is due to the fungus-form mentioned, and this view is sup- 

 ported by the fact that similar forms are known to produce simi- 

 lar diseases in some other plants. This fungus appears to be able 

 to develop also a saprophyte on old lettuce-leaves and other vege- 

 table refuse, and may thus survive a long interval between two 

 crops of lettuce, resuming its parasitic habits when the opportu- 

 nity is afforded. 



From what has been said, it is evident that careful and thor- 

 ough treatment is essential to the control of the disease in ques- 

 tion; and the nature of the crop limits this treatment to the re- 

 moval of all sources of infection. All affected lettuce plants 

 should be at once removed wholly from the house and destroyed 

 by burning. For this pui-pose the boiler furnace is conveniently 

 at hand. All dead leaves or other refuse should be often scrupu- 

 lously cleaned up and burned, so that no breeding places may be 

 left for the fungus. A house which has been very badly infested 

 by the disease should be thoroughly cleaned, whitewashed, or 

 painted, and supplied with fresh soil before a new season's opera- 

 tions are begun ; and one may then expect, with the observance 

 of the above described hygienic precautions, to be able to enjoy 

 comparative freedom from loss from this cause. 



The powdery mildew of the cucumber is due to the presence of 

 a fungus which has been long known, but which has not been 

 heretofore reported as occurring in America, so far as can be 

 learned. It has been received during the past season, on leaves of 

 greenhouse cucumbers, from Dr. Jabez Fisher of Fitchburg and 

 from Professor L. H. Bailey of Cornell University. The fungus, 

 as has been said, attacks the leaves, on whose upper surfaces it 

 forms at first rounded spots, which appear like blotches of a white 

 powder. These spots gradually enlarge and become confluent 

 until the leaf is practically covered. Those parts of the leaf 

 which are attacked soon tui'n yellow, and finally become dead and 

 dry. Under favorable conditions the disease spreads quite rapidly 

 and is very destructive. 



The fungus which causes the trouble is known as Oidium ery- 

 siphoides Fries, var. Oucurbitarum, and is the conidial or summer 

 spore stage of one of the fungi known as " powdery mildews.'' 

 It is impossible to say certainly to which of the perfect or winter 

 spore forms of the gi'oup it belongs. 



It has been found by Professor Bailey and by Dr. Fisher that 

 the fungus may be kept in check by frequent spraying with a so- 

 lution of sulphide of potassium (liver of sulphur) in water. The 

 proportion usually given is one ounce of the sulphide to two gal- 

 lons of water, but both Dr. Fisher and Professor Humphrey have 

 found this solution injurious to the foliage and the young cucum- 

 bers. A preparation containing an ounce to three gallons is cer- 

 tainly strong enough, and one with an ounce to four gallons is 

 probably so. 



As recommended for the lettuce disease, a house in which this 

 fungus has been troublesome should be thoroughly cleaned and 

 fumigated before the next season's crop is started. 



THE AGRICULTURAL PRODUCTS OF MADAGASCAR. 



M. d'Anthouaed, Chancellor of the French Residency at An- 

 tananarivo, has recently made to the Fi-ench Government an in- 

 teresting report upon the economic condition of Madagascar, a 

 translation of which appears in the Journal of the Society of Arts 



