906 CONCRETE 



from the coarsest to those having from forty to forty-five teeth to the inch. The 

 gage-saw or gaae-vid is used to make the teeth square and of one depth. The saw is 

 frequently made with a loose back, like that of ordinary back-saws, but much wider, 

 so that for teeth $, $, J inch long, it may shield all the blade except , , inch of its 

 width respectively, and the saw is applied until the back prevents its further progress. 

 Sometimes the blade has teeth on both edges, and is fixed between two parallel slips 

 of steel connected beyond the ends of the saw blade by two small thumb-screws. 

 After the teeth of combs are cut they are smoothed and polished with files, and by 

 rubbing them with pumice-stone and tripoli. Holteapffel, 



COMBINING NUMBERS AND CHEMICAL COMBINATION-. 



Constancy of composition is one of the most essential characters of chemical com- 

 pounds ; by which is meant that any particular body, under whatever circumstances 

 it may have been produced, consists invariably of the same elements in identi- 

 cally the same proportion ; the converse of this is not, however, necessarily true, 

 that the same elements in the same proportion always produce the same body. See 

 ISOMERISM. 



But not only is there a fixity in the proportion of the constituents of a compound ; 

 but also, if any one of the elements be taken, it is found to unite with the other 

 elements in a proportion which is either invariable, or changes Only by some very 

 simple multiple. 



The numbers expressing the combining proportions of the elements are only relative. 

 In England it is usual to take hydrogen as the starting-point, and to call that number 

 the combining number of any other element which expresses the proportion in which 

 it unites with one part by weight of hydrogen : and these numbers are now likewise 

 adopted on the Continent, although in France the combining numbers are sometimes 

 referred to oxygen, which is taken as 100. It is obvious that, whichever system is 

 used, the numbers have the same value relatively to each other. 



These combining numbers would have but little value if they expressed nothing 

 more than the proportion in which the elements combine with that body arbitrarily 

 fixed as the standard ; but they also represent the proportions in which they unite 

 among themselves, in the event of union taking place. Thus, not only do 8 parts of 

 oxygen unite with one of hydrogen, but also 8 parts of oxygen unite with 39 of 

 potassium, 23 of sodium, 100 of mercury, 108 of silver, &c. It is in virtue of this law 

 that the combining proportions of many of the elements have been ascertained. Some 

 of them do not combine with hydrogen at all, and in such cases the quantity which 

 unites with 8 parts of oxygen, or 16 of sulphur, &c., has to be ascertained. See 

 ATOMIC WEIGHTS; EQOTVA:LENTS. H.M.W. 



COMBUSTIBLE. (Eng. and Fr. ; Brennstoff, Ger.) Any substance which, 

 exposed in the air to a certain temperature, consumes with the emission of heat 

 and light. All such combustibles as are cheap enough for common use go under the 

 name of Fuel. Every combustible requires a peculiar pitch of temperature to be 

 kindled, called its accendible point. Thus phosphorus, sulphur, hydrogen, carburetted 

 hydrogen, carbon, each takes fire at successively higher heats. See FLAME ; FTJEL. 



COMBUSTION. (Eng. and Fr.; Verbrennung, Ger.) The phenomena of the 

 development of light and heat from any body, as from charcoal combining with the 

 oxygen of the air, from phosphorus combining with iodine, and from some of the 

 metals combining with chlorine. Combustion may be exerted with very various 

 degrees of energy, We have a low combustion often established in masses of vegetable 

 matter ; as in haystacks, or in heaps of damp sawdust. In these cases, the changes 

 going on are the same in character, only varying in degree, as those presented by an 

 ordinary fire, or by a burning taper oxygen is combining with carbon to form carbonic 

 acid. The heat thus produced (the process has been termed by Liebig Eremacausis), 

 increasing in force, gives rise eventually to visible combustion. 



Cases of spontaneous combustion are by no means uncommon. Some years since, 

 the editor .investigated the conditions under which H. M. ships the ' Imogen ' and 

 4 Talavera ' were burnt in Devonport Dockyard, and ho was enabled to trace the fire 

 to a large bin, in which there had been allowed to accumulate a mass of oiled oakum, 

 pieces of old flannel covered with anti-attrition, sawdust, shavings, and the sweepings 

 of the painters', wheelwrights', and some other shops. 



The subject of combustion belongs to Watts's 'Dictionary of Chemistry,' where it is 

 fully treated. See SPONTANEOUS COMBUSTION. 



CONCRETE. The name given by architects to a compact mass of pebbles, 

 sand, and lime, cemented together in order to form the foundation of buildings. 

 Semple says that the best proportions are 80 parts of pebbles, each aboiit 7 or 8 ozs. 

 in weight, 40 parts sharp river sand, and 10 of lime ; the last is to be mixed with 

 water to a thinnish consistence, and grouted in. It has been found that Thames 

 ballast, as taken from the bed of the river, consists of nearly 2 parts of pebbles to 1 of 



