€ht popular Science i^elos 



AND 



BOSTON JOURNAL OF CHEMISTRY. 



Volume XXII. 



BOSTON, MAY, 1888. 



Number 5. 



CONTENTS. 



Familiar Scibncb. — Dew and Frost 66 



The Common Dandelion 66 



The French Academy of Sciences 67 



The Diffraction of Sound 68 



Scientific Brevities 68 



I'BACTICAL Chemistbt AND THE Abts. — Glass and Por- 

 celain Painting 69 



New Methods in Analysis 69 



Liquid Amalgam 69 



Surface Condensation of Glass 69 



Chloride of Nitrogen 70 



Aahestos for Filtration 70 



A Substitute for Hydraulic Cement 70 



A Test for Saccharin 70 



I'ractical Recipes ■■ 70 



Railroad Notes 70 



Home, Farm, and Garden. — The Oheraistry of the Dairy. 71 



Teat 71 



The Bird-Foot Violet 71 



What Flowers win grow In the Shade? 72 



A llspice 72 



Horticultural Hints 72 



Editorial. —Quantivalence 73 



The Snow as a Bird-Catcher 74 



Heriiu Letter 74 



How Birds learn to Sing 75 



Astronomical Phenomena for May, 1888 75 



Meteorology for March, 1888 76 



Questions and .Answers 76 



Literary Notes 76 



Medicine and Pharhact. — Man ass Steam Engine 77 



Simple Sphygmographs 77 



The Coca-Plant 77 



The Functions of the Supra-lienal Capsules 78 



Medicine for the Imagination 78 



Monthly Summary of Medical Progress 78 



Pharmaceutical N otes 79 



Select Formulffi for Dentifrices 80 



Medical Memoranda 80 



Publishers' CoLrMH 80 



familiar S^tienn. 



DEW AND FROST. 



If, after a clear, cool night, we observe the 

 surface of the ground, we often find it cov- 

 ered with moisture, as if it had rained during 

 the night. This familiar phenomenon, Icnown 

 as the fall of the dew, is dependent upon some 

 verj- interesting and important scientific prin- 

 ciples. 



Air possesses the properly of holding a 

 certain amount of water vapor, the quantity 

 depending upon the temperature. If, at a 

 temperature of 60°, for instance, a cubic foot 

 of air held as manj- grains of water in the 

 form of vapor as it was possible for it to do, 

 it would be said to be saturated, just as a 

 quantity of water would be saturated with salt 

 when no more of that substance could be dis- 

 solved in it. Now, if the temperature of the 

 air was increased to 70°, its power of absorb- 

 ing water vapor would be increased ; and if 

 it was in contact with water or a moist sub- 

 stance, evaporation would commence, and con- 

 tinue until the air was saturated once more. 

 If, on the contrary, the temperature of the 

 saturated air was lowered, it would become 

 more than saturated, and the excess of water 

 vapor would be precipitated out in the form of 

 flnel}' divided particles. 



One of the most familiar examples of this 

 phenomenon is shown by our breath on a cold 

 morning, when the water vapor in the warm 



moist air from our lungs, on passing into the 

 colder outside air, condenses in the cloud-like 

 form so frequentlj- observed. When this 

 takes place in nature, on a large scale, by a 

 cold and a warm body of air coming together, 

 fogs, clouds, or rain are produced. 



It is a curious fact that the water thus pre- 

 cipitated from a state of vapor is not in the 

 form of little drops, but of hollow vesicles, as 

 the}' are termed, like minute soap-bubbles. 

 This peculiar form is common to condensed 

 water vapor, whether in the form of steam 

 escaping from a teakettle or boiler, or the fogs 

 and clouds which float in the air. It is only 

 when the vesicles unite together that ordinary 

 drops are formed. This vesicular structure 

 probably helps in maintaining the clouds at 

 their high elevation. 



Whatever the quantity of vapor in the air, 

 there is a certain temperature at which the 

 vapor begins to precipitate, known as the 

 dew-point. If a goblet filled with ice-water 

 is brought into a warm room, the air imme- 

 diately surrounding it is cooled below the 

 dew-point, and a copious deposit of water is 

 formed upon its sides. This water does not 

 ooze out through the glass, as some people 

 suppose, but is squeezed out of the air, as it 

 were, by the cooling effect of the ice. 



We thus see that it is only necessary to cool 

 the air sufficiently to obtain a copious deposit 

 of moisture, and if the temperature of the 

 ground should fall low enough, dew would 

 certainly be precipitated upon it ; but it is 

 noticed that dew falls upon warm nights, when 

 the temperature of the air is far above the 

 dew-point. What, then, causes the cooling of 

 the earth upon which the dew is so abundantly 

 deposited ? 



This is best explained by the principle of 

 radiation. If we hold our hand at some dis- 

 tance from a block of ice, wc shall feel the 

 cold ver}- perceptibly-. The hand radiates its 

 heat towards the ice, which absorbs it so rap- 

 idly that the temperature of the hand is low- 

 ered, and a sensation of cold is felt. Now, 

 just above the comparatively thin stratum of 

 air which surrounds our earth are the regions 

 of space where the temperature is far lower 

 than any with which we are acquainted ; and 

 it is to these cold regions that the heat of the 

 earth, the grass, and other bodies is radiated 

 in the clear summer nights, after the heat of 

 the sun is withdrawn. As the}' become cooled 

 hy radiating away their heat, thoy reduce the 

 temperature of the air surrounding them below 

 the dew-point, and the dew is thus precipitated, 

 not hy '' falling," as the common phrase im- 

 plies, but from the air immediately in contact 

 with the object on which it is deposited. 



This theory of the formation of dew also 

 explains many of the accompanying phe- 

 nomena. A gentle wind favors the deposi- 

 tion of dew, as it brings fresh supplies of 

 moisture-laden air into contact with the cold 

 earth or foliage. A brisk wind, on the con- 

 trary, prevents its formation, as the air does 

 not remain long enough in contact with the 

 cool bodies to deposit its moisture. No dew 

 falls on cloud}' nights, because the clouds act 

 as a screen, and prevent the radiation from the 

 earth's surface to the colder regions beyond. 



It is noticed that the dew is not deposited 

 in equal quantity upon different substances. 

 A bright piece of metal exposed on the ground 

 will remain quite dry, while the surrounding 

 grass will be loaded with moisture. This is 

 due to the fact that different substances pos- 

 sess different capacities for radiation. In gen- 

 eral, it may be said that dark-colored, rough 

 bodies are good radiators, while smooth, bright 

 substances are poor ones. Air also is a com- 

 paratively poor radiator, or else it would cool 

 itself down below the dew-point, and form 

 fogs, much more frequently than at present. 



Hoar-frost is, of course, simply frozen dew. 

 In the spring and autumn the temperature of 

 the earth may not only fall below the dew- 

 point of the air, but even below the freezing- 

 point of water. The flocculent form in which 

 hoar-frost is often deposited shows that the 

 ice has been formed directly from the con- 

 densing vapor, without passing through the 

 intermediate stage of water. In some [)arts 

 of India ice is manufactured b}- this means : 

 shallow earthenware dishes of water are ex- 

 posed to the clear sky at night, and the water 

 is often frozen even when the temperature of 

 the air is as high as 50° F. Early crops are 

 often frozen in the same wa}' ; and farmers 

 sometimes build bonfires in their fields at night, 

 so that the smoke shall form an artificial cloud, 

 and prevent the radiation and consequent 

 cooling. Finally, the beneficial effect of the 

 heavy dews of midsummer to the crops, when 

 parched with drought, is of great value, and 

 they replace to no small extent an insufficient 

 rainfall. 



[Original in Popular Science New8.\ 

 THE COMMON DANDELION. 



BY FREDERICK LEROY SARGENT. 



In the various names which the dandelion has 

 received, we see expressed, for the most part, either 

 a reference to the tooth-like recurved lobes of the 

 leaves (Fig. 1), or an allusion to the medicinal 

 properties of the plant. Thus, our English name is 

 a modified form of the French dent de lion, meaning 

 lion's tooth, and in German we have the same idea 

 expressed in Lowenzahn. Fifty years ago this 

 plant appeared in the botanies as Leonlodon laraxi- 



