November i, 1903.] 



THE INDIA RUBBER WORLD 



45 



SPECIFIC GRAVITY IN RUBBER COMPOUNDING. 



THE ratio of bulk to weight is of great practical import- 

 ance in the rubber industry, because it controls the 

 number or pieces or feet per pound obtainable from 

 any given stock. This relation of bulk to weight is de- 

 pendent on the specific gravity of the material. Its determina- 

 tion presents a constantly recurring problem that the rubber 

 factory superintendent must solve by some means or other if 

 he is to work to the best advantage. The following explana- 

 tion, it is hoped, will make clear to any who may be unfamiliar 

 with the term what is meant by " specific gravity." 



Every material whether solid, liquid, or gaseous has weight 

 or density dependent on its nature or composition. These 

 weights vary through a wide range from the very heavy solids 

 to the lightest gases, taken bulk for bulk. The specific gravity 

 of any substance is the particular ratio of its weight to that of an 

 equal bulk of another substance, taken as a standard or unit 

 weight. For all solids and liquids the standard substance of 

 unit gravity is distilled water at the temperature of 62" Fahren- 

 heit. For gases the standard is hydrogen gas at the atmos- 

 pheric pressure of the sea level. 



The following table gives the specific gravities of a few com- 

 mon substances and will be convenient for reference. It will 

 be understood that the figures express averages and are near 

 enough for practical purposes of technical work. The value 

 for each substance is given in terms of water as unity : 



SPECIFIC GRAVITIES OF SOME COMMON SUBSTANCES USED IN RUBBER 

 COMPOUNDING. 



Antimony sulphide 4.6 



Asbestine 2.6 



Asphaltum 1.3 



Barytes 4.5 



Caoutchouc o 94 



Fossil flour 1.8 



Graphite 2.0 



Gutta-percha o 99 



Iron oxide 2.0 



Kaolin 2.2 



Lampblack 0.2 



Litharge 9.3 



Lithopone 3.6 



Magnesia 3.4 



Plaster of Paris 2.9 



Pumice 2.2 



Red lead 8.5 



Rosin. 1.1 



Sublimed lead 8.0 



Sulphur 2.0 



Talc 2.7 



Tar 1.0 



Vermilion 8.1 



Whiting 2.8 



White lead 6.2 



Zinc oxide 5.6 



The method of determining specific gravities of solids de- 

 pends on the fact that any substance immersed in water loses 

 weight equal to the weight of the volume of water which it dis- 

 places. The means of ascertaining specific gravities vary some- 

 what according as the substance under examination is solid, 

 liquid, or a gas. Only those methods will here be given that 

 relate to solids and liquids, since they comprise the extent of 

 the problem for rubber workers. The density of any substance 

 bears the same proportion to the density of water as the weight 

 of the substance bears to the weight of its bulk of water. 

 Hence if the weight of the body, in air, is divided by its loss of 

 weight, when weighed in water, this quotient will represent the 

 specific gravity or comparative density of the body. 



The apparatus illustrated and described in this article is de- 

 signed to afford a ready means of weighing materials in air and 

 water, thus obtaining the data for determining the specific 

 gravity. 



Every chemical balance is provided with a hook at either 

 end of the beam for use in suspending a sample to permit its 

 weight to be taken in water, the glass containing the water be- 

 ing placed on a support standing on the floor of the balance 

 case and astride the scale pan. 



The Jolly spiral balance, so called from its inventor, is es- 



pecially useful for obtaining rapidly the specific gravities of 

 minerals and rubber samples and is really indispensable in rub- 

 ber works. It consists of an upright supported on a heavy iron 

 base, which is provided with leveling screws to adjust the in- 

 strument plumb. Extending the full length of one side of this 

 upright is a mirror upon which is engraved a fine scale of equal 

 parts arranged decimally. Sliding on the upright is a small 

 platform for supporting a glass of water, and adjustable at any 

 height by a thumbscrew. Sliding into the upright is a light 

 adjustable wooden rod carrying an arm arranged for holding 

 one end of the weighing spiral of wire which at its lower end 

 hooks to the pans, of which there are two connected together. 

 Three spirals of various degrees of tension are provided with 

 the instrument to regulate its sensibility to heavy, medium, or 

 light materials. Thus set up, as shown in the illustration, with 

 the pans suspended from the medium spiral, allow the lower or 

 glass pan to hang freely in a glass filled with clean water. It is 

 proper to use distilled water, of course, as coming nearer the 

 scientific standard. If such water is not available, clean cool 



WESTPHALE S BALANCE. 



water, that has been previously boiled to expel the dissolved 

 air, will answer very well. 



To make a specific gravity determination, begin by adjusting 

 the glass of water at such height that the lower pan will be im- 

 mersed to some point above where its supporting wires meet. 

 Allow the pans hanging free in this way to come to rest, and 

 note the reading on the scale of the height of some fixed point, 

 as the top of the white bead. The scale is engraved on a mirror 

 in order that a level reading may be taken by sighting the point 

 selected for reading with its reflection. Every reading must be 

 made from one reference point. Record this reading taken 

 with the pans empty. Then place in the upper pan a small piece 

 of the rubber or other material to be tested, of suitable size (and 

 any shape). Again adjust the level of the glass so that the pans 

 may hang free and with the lower pan immersed as before. 

 When equilibrium is established note the second reading of the 

 same reference point and record. In precisely similar way de- 

 termine the reading of the reference point again with the sam- 

 ple in the lower pan immersed. Care must be taken to free the 

 sample of all adhering air bubbles which would otherwise falsi- 



