DIVING BELL 61 



all may be produced by the action of high temperatures on more or less compli- 

 cated bodies. Much has already been done, but the facts at present accumulated 

 relate merely to the superficial and more salient substances. On penetrating further 

 below the surface far more valuable and interesting facts will came to light. C.G.W. 

 DISTIIiliED WATER. Water converted into vapour by heat, and subsequently 

 condensed. See WATER. 



DITTANY. The root of the Diciamnus Fraxinella, a plant belonging to the 

 Kue order, or Rutacece, formerly much used in medicine. 



DISSOCIATION. The resolution of certain compounds at a high temperature 

 into their constituents ; the elements of which they are composed being capable of 

 existing together under these conditions, without entering into combination. Thus, 

 carbonic oxide is dissociated at a high temperature, oxygen being ^set free and com- 

 bining with unaltered carbonic oxide to form carbonic acid gas, whilst free carbon is 

 deposited. According to Mr. I. Lowthian Bell, this decomposition is of much im- 

 portance in the reactions which accompany iron smelting in the blast-furnace. As 

 dissociation is a purely physical phenomenon, hardly within the limits of this Dic- 

 tionary, we may refer the reader to 'Lemons sur la Dissociation' (1864), by M. Henri 

 Sainte-Claire Deville, who has made this subject peculiarly his own. 



DIVI-DIVI, or Libi-Davi, is the pod of a leguminous shrub, the Casalpinia 

 coriaria, which is indigenous to Jamaica, and some parts of South America. Mr. 

 Kootsey obtained a mean produce of 6'625 grs. of leather from 60 grs. of divi-divi, 

 while the same quantity of the best Aleppo galls yielded only a mean produce of 

 4-625. It appears, too, from Sir Humphry Davy's estimate, that 60 grs. of divi-divi 

 contain 3*0475 grs. or 5'079 per cent, of tannin, and 60 grs. of galls, 2-12704 grs., or 

 3-450 per cent. Sixty grs. of oak bark yielded only 1'75 grain of leather ; whence it 

 follows that it contains but 0*805 of a grain of tannin to the drachm, or not more 

 than 1-34166. 



It has been tried as a dye instead of galls or sumach, but its nse for this purpose is 

 almost entirely abandoned. See LEATHER. 



DIVING BEIiIi. As it is frequently desirable to raise objects from the bottom of 

 the sea or rivers, and to lay the foundation of piers and similar structures, some con- 

 trivance was desired to enable man to descend below the water, and to sustain himself 

 while there. The first method adopted was the very simple one of letting down a 

 heavily-weighted bell vertically into the water. As the bell descended, the air got 

 overpressed, and the water rose in the bell, but never to the top, and within that 

 space the man was sustained for some time. The air, however, was vitiated by the 

 process of respiration, and the man had to be drawn up. It is curious to find that as 

 early as 1693 a very complete system of diving without a bell was devised, as the 

 following quotation will show : 



A. D. 1693. 'William and Mary, by the Grace, &c. &c. Whereas John Stapleton, 

 gentleman, hath by his great study and expence invented a new and extraordinary 

 engine of copper, iron or other mettal, with glasses for light joints, and so contrived 

 as to permit a person enclosed to move and walk freely with under water, and yet so 

 closely covered over with leather as sufiicientlyto defend him from all the jumpes of it. 

 Also invented a way to force air into any depth of water, whereby the person in the 

 aforesaid engine may be supplied with a continual current of fresh aire, which not 

 only serve him for respiracon, but may alsoe be useful for continuing a lamp burning, 

 which he may carry about with him in his hand. . . . Likewise a way to make the 

 same again serviceable for respiracon, and by continually repeating the operacon, a 

 man may remain a long time under water, in either of the said engines, without any 

 other air than the sayd engines do contayne, whereby he shall be preserved from suffo- 

 cation if any extraordinary accident should interrupt the current of fresh air afore 

 menconed.' Letters Patent. Rolls Chapel. Edited by Sennet Woodcroft. 



The defects were many in this apparatus, and Dr. Halley invented a bell the object 

 of which was to remedy them. 



Dr. Halley's bell was of wood coated with lead, and having strong glass windows 

 above, to allow the passage of light to the diver. In order to supply air, a barrel was 

 taken with an open hole in the bottom, and a weighted hose hanging by, and fitting 

 into a hole at the top. From this barrel the air of the bell was supplied as frequently 

 as it became vitiated, the barrels of air being sent down from above. Spalding im- 

 proved upon Halley's bell, and again Friewald made some improvements on Spalding's, 

 but in principle these bells were all alike. The modern bells are usually large and 

 strong iron bells, with windows in the upper part. By means of an air-pump, placed on 

 the surface, air is sent down to the divers in the bell, and the vitiated air is as regularly 

 removed from the bell by other tubes through which it escapes. These diving bells 

 are lowered by means of cranes, and are moved about in the water by those above, 

 signals being given by the men below. The difficulty of moving this machine, renders 



