DET 



DEW 



as when nitre is thrown on red-hot 

 coals. 



DEFLECTION. A terra in optics. 

 When a thin opaque body is placed in 

 the course of a ray of light, the ray 

 is bent out of its straight direction. 

 The phenomenon is also called dif- 

 fraction. 



DEGLUTITION. The act of swal- 

 lowing. 



DEHISCENT. A botanical term 

 signifying the bursting open, when 

 dry, of seed vessels. 



DELIQUESCENT. Saline sub- 

 stances which absorb so much moist- 

 ure from the air as to become fluid 

 are called deliquescent. 



DELPHINIA. A vegetable alka- 

 loid from Stavesacre, or Delphinium. 



DELPHINIC ACID. An oily acid, 

 obtained from whale od, having a 

 rancid smell. 



DENDROMETER {iromdevdpov, a 

 tree, and /j.eTpov,ameasurc). An instru- 

 ment like an immense pair of com- 

 passes, to measure the height and 

 the girth of trees, for estimating the 

 amount of timber. 



DENTATE (from dens, a tooth). 

 Toothed. 



DENTIROSTERS. Birds having 

 a tooth-like notch on each side of the 

 upper mandible. They are very ra- 

 pacious. 



DEOBSTRUENT. A medicine 

 given to remove any obstruction in 

 the bowels, &c. 



DERBYSHIRE SPAR. Fluor 

 spar, or fluoride of calcium. 



DESPUMATION. The act of 

 skimming the scum from any heated 

 fluid. 



DESTRUCTIVE DISTILLA- 

 TION. The heating of bones, wood, 

 coal, &c., in iron vessels, at a high 

 temperature, to produce peculiar sub- 

 stances. From green wood, vinegar 

 and wood tar ; from bones, impure 

 ammonia ; from coal, gas, coal tar, 



DETERGENTS. Medicines which 

 remove impurities and cleanse sores. 



DETONATION. In chemistry, ex- 

 plosions on a small scale 



DETRITUS. The broken and 

 pounded remains of rocks. 



DEUTOXIDE, BINOXIDE. A 

 compound acting as a base, which 

 contains two atoms of oxygen 



DEVON CATTLE. Esteemed for 

 draught. See Cattle. 



DEW. The deposite of water from 

 the air produced by cold ; it becomes 

 frost when the cold is below 32° Fah- 

 renheit. As soon as the sun sets, the 

 heat imparted to the earth begins to 

 fall by radiation into space ; if clouds 

 be present, the heat is mostly return- 

 ed again ; if in a clear sky, it is lost, 

 and the earth's surface chilled. The 

 cold of the surface chills the air lying 

 above it, and causes a deposite of its 

 water ; hence the dew. Those bod- 

 ies which cool quickest receive most 

 dew ; black soils more than light-col- 

 oured ; rough surfaces more than pol- 

 ished. Dew, therefore, only falls on 

 clear nights, and frost observes the 

 same rule. When the atmosphere is 

 loaded with water, the cooling of a 

 few degrees is sufficient to form dew ; 

 hence most falls near rivers and 

 streams. 



DEW POINT. The temperature 

 at which dew falls. It is a very im- 

 portant fact in meteorology, and ea- 

 sily ascertained. Place in a clean 

 wine-glass, half full of water, a little 

 ice, untd a mist of dew is seen on the 

 outside ; remove the ice without wet- 

 ting the surface, plunge a thermome- 

 ter into the water, and observe the 

 temperature as the mist disappears : 

 the degree marks the dew point. 

 This IS the simplest way, and as 

 good as any. The difference in de- 

 grees between the air and dew point 

 is called the drying power, and shov.'s 

 how much more moisture the air will 

 take. When they agree, the air is 

 filled or saturated with moisture. The 

 amount of water in the atmosphere is 

 connected with the probability of rain, 

 the growth of plants, the occurrence 

 of mildew, rust, &.c., and should be 

 measured by the farmer on important 

 occasions as a means of ascertaining 

 its relation to these points. 



DEWBERRY. The creeping 

 blackberry, which see. 



DEWLAP. The fold of skin be- 

 low the neck of cattle. 



227 



