46 



THE INDIA RUBBER WORLD 



[November i, 1903. 



fy the reading. Note the third reading and the data will be 

 ready for calculation. These readings represent, in terms of 

 spaces on the scale, (1) the weight of the pans unloaded; (2) 

 the weight of the pans and substance in air; (3) the weight of 

 the pans and substance in water. 



The difference between the first and second readings stands 

 for the weight of the sample in air. The difference between the 

 second and third readings represents the loss of weight of the 

 sample in water. Divide the weight in air by the loss of weight 

 in water and the result will express the specific gravity. For 

 solids lighter than water it will be found necessary to 

 close the wires of the lower pan more or less around the 

 sample to keep it immersed. 



Another and simpler instrument for obtaining specific 

 gravities of solids is known as the Nicholson hydrometer. ^_ 

 This is made of thin sheet metal of hydrometer form, and 

 provided with a set of small weights. It is inexpensive 



and accurate, but not as convenient 



to use as the Jolly balance. Above 



and below the body of the hydrom- 

 eter are pans for holding the sample. 



On the stem is a reference mark to 



which point the instrument is always 



sunk in the jar of water before each 



reading is taken. Briefly described, 



its use is as follows : * Let a/j be the 



weight required to sink the instru- 

 ment to the mark on the stem, the 



weight of the instrument being w ; 



to take the specific gravity of any 



solid substance place a portion of it 



weighing less than iv x , in the upper 



pan, with such additional weight, 



say w t , as will cause the instrument 



to sink to the zero mark. The weight 



of the substance, in air, is then w^ — 



w a . Next transfer the substance to 



the lower pan, and again adjust with 



weight w i to the zero mark. The 



loss of weight of the substance in 



water is then a/ 4 — w t . Therefore the 



specific gravity is obtained by this 



formula : 



ORDINARY HYDROMETER. 



Specific gravity=- 



w l —w a 

 For materials in the form of powder the 

 specific gravity bottle is used. This is of 

 various forms, but is essentially a small 

 flask provided with a reference mark on 

 the neck. A fine chemical balance is nec- 

 essary to make the weights and the pro- 

 cedure is as follows for solids heavier than 

 water:* Weigh the flask filled to the ■ 



mark with water, then place the substance, jolly's spiral balance. 



of known weight, in the flask, fill to the mark with water, and 

 weigh again. 



Turning to the consideration of the means of obtaining the 

 gravities of liquids such as acids, oils, naphtha, etc., we have 

 the various forms of hydrometers and the Westphale's balance. 

 There are many specially designed hydrometers adapted to the 

 requirements of certain industries, but in principle they are all 

 alike. They consist of a weighted glass bulb sinking the in- 

 strument upright in the liquid and reading the degree, or actual 

 specific gravity, by means of graduations on the stem. The or- 

 dinary Beaume' hydrometers are those in general use. Two in- 

 struments are required, one weighted and graduated for liquids 

 heavier than water, and one for those lighter than water. 

 The Beaume scale of " degrees " is arbitrary and to as- 

 certain the specific gravities a table must be consulted. 

 For ordinary trade purposes the Beaume degree is used 

 and is all that is required. The Westphale's balance is 

 adapted to either light or heavy liquids and by its aid the 

 gravities are read direct from the weights used without 

 calculation. It is also convenient 

 when only small samples of liquids 

 are available for examination. The 

 balance is so adjusted by the mak- 

 ers that the glass bob will balance 

 the counter weight on the opposite 

 arm when hanging in air. When 

 suspended in any liquid a buoying 

 effect, dependent on the gravity of 

 the material throws the instrument 

 out of balance. The equilibrium is 

 reestablished by means of a set of 

 rider weights. Reading the position 

 on the beam of the weights in the 

 order of their size gives at once the 

 specific gravity sought without cal- 

 culation. 



It should be noted that specific 

 gravity is not to be taken as a test 

 for quality as applied to rubber 

 stocks, but should be considered 

 simply as a guide to the economy of 

 the stock. An other practical appli- 

 cation is found in estimating the 

 weight of a proposed article of solid 

 hydrometer. stock when its cubical contents is 

 known. The weight for water of the cub- 

 ical contents is ascertained by multiplying 



by 252.5, the weight in grains of one cubic 



inch of water. This product multiplied 

 by the specific gravity of any material will 

 give the weight of the object in that 

 material. Thus an article of 10 cubic 

 inches volume would weigh, if made of a 

 rubber stock of 1.85 specific gravity : 

 10X252. 5Xt.35 = 4&7i-25 grains = 10% oz. 



The calculation will be ; 



(Weight of substance in air)+(weight of 

 flask and water) — (weight of flask 

 _and water and substance) 

 ""(weight of substance in air). 



It will be unnecessary to discuss the methods employed in 

 determining the gravities of substances soluble in water or of 

 gases. 



• From Bailey's " Chemists' Pocket Book." 



A petition has b een addressed to the governor general of 

 French West Africa by the rubber trade of Bordeaux, asking 

 that measures be taken to prevent the exhaustion of the native 

 rubber vines ol the Soudan and adjacent districts, to encourage 

 planting, and to promote improvement of the quality of the 

 present production. It is pointed out that the quality of the 

 Conakry rubbers is satisfactory, and that, by the adoption of 

 similar methods of preparation, other French colonial rubbers 

 could be made as good. Credit is given in this regard to the 

 action of the governor of French Guinea. The petition is 

 signed by twenty rubber brokers, importers, experts, etc. 



