448 



NATURE 



[March io, 1892 



attached. After about ten or fifteen minutes, crystals begin to 

 make their appearance in the U-tube ; the oxide then rapidly 

 collects in the form of a waxy mass. The best refrigerator for 

 the U-tube is pounded ice ; if salt is mixed with it, the oxide 

 condenses so rapidly in the first cooled portion of the condenser 

 as to form abridge and stop the opeiation until it is melted 

 down. When nearly half of the phosphorus is burnt, the brass 

 condenser is warmed by pouring in water heated to about 70°- 

 80° C, and the condenser is maintained at this temperature 

 until about three-quarters of the phosphorus has been burnt, 

 when the operation is stopped by slowly turning off the water 

 working the pump. 



When phosphorus is burnt under these conditions, three 

 oxides are produced ; more or less of the red suboxide P4O is 

 always deposited in the immediate neighbourhood of the 

 burning phosphorus, a certain amount of pentoxide is formed 

 and retained in the glass tube beyond the seat of combus- 

 tion and in the brass condenser, and phosphorous oxide is 

 produced in large quantity, and, being considerably more vola- 

 tile, is carried forward to the cooled condenser, any which may 

 be deposited in the brass condenser during the earlier stages of 

 the combustion being cairied along into the U-tube in the 

 current of escaping nitrogen when the warm water is introduced 

 into the brass condenser. Scarcely a trace of pentoxide escapes 

 through the glass wool filtering plug, the product in the U-tube 

 being almost pure phosphorous oxide. In the course of five 

 hours three such charges of phosphorus may be burnt out and the 

 total phosphorous oxide produced, which should amount to at 

 least twenty grams, can be condensed in the same U tube, the 

 product from each charge being melted down into the bottle so as 

 to prevent choking of the tube. In order to free the product from 

 any traces of impurity, it should be distilled in a slow current of 

 carbon dioxide, when it condenses in the receiving tube as an 

 absolutely clear liquid which soon solidifies to a snow-white 

 mass of crystals. The tube should be at once sealed and, for a 

 reason which will be found under the Action of Light, kept in the 

 dark. 



Molecular Comfosiiion of Phosphorous Oxide. 

 Quantitative analysis of the substance whose properties and 

 mode of preparation have just been described of course yields 

 numbers which agree with the empirical formula PoOg. But 

 as the oxide is volatile it w as of the first importance to determine 

 its vapour density, with the view of obtaining information 

 regarding its molecular weight. This determination was the 

 more interesting from the fact that Prof Victor Meyer had 

 previously found that the analogous oxides of arsenic and anti- 

 mony gave vapour densities corresponding to the double mole- 

 cular formulae AS4O6 and Sb406, and also from the fact that 

 the molecule of phosphorus itself is found to contain four atoms. 

 The vapour density was determined by Hoffmann's well-known 

 method in the Torricellian vacuum at the temperatures of boiling 

 amyl alcohol (132°), oil of turpentine (159°) and aniline {184°). 

 The numbers obtained from several such determinations are in 

 perfect accordance with the molecular weight corresponding to 

 the double formula P4O6. This result has been fully confirmed 

 by a determination of the molecular weight by the totally 

 different method of Raoult, which depends upon the degree of 

 lowering of the freezing point of a solvent, in this case benzene, 

 by the introduction of a small quantity of the substance under- 

 going investigation. 



Hence phosphorous oxide must be symbolized by the formula 

 P4O6 and not PoOg, phosphorus thus resembling its family 

 relatives arsenic and antimony in the nature of its lower com- 

 bination with oxygen. 



Physical Properties of Phosphorous Oxide. 



The specific gravity of the solid oxide at 21° C, compared with 

 water at 4° is 2'135, and that of the liquid oxide at 24°-8 is 

 I -9358. Hence there is about nine per cent, of contraction 

 upon the passage of the liquid into the, solid state. 



A somewhat interesting result has been obtained from the 

 determination of the specific volume, that is, the number 

 obtained by dividing the molecular weight by the density at the 

 boiling point. The actual density, of course, cannot be experi- 

 mentally determined at the temperature of ebullition, but by 

 making a careful determination of the rate of expansion and 

 knowing the density very precisely at some lower temperature 

 the density at the boiling point can be calculated. The value 

 thus found for the specific volume was 130-2. Now, phosphorus 

 is known to possess two specific volumes ; one in the state of 



combination (as determined from its halogen derivatives) and 

 which is somewhere about 25*3, and another when in the free 

 state, which is approximately 20 "9. Oxygen, too, is usually 

 supposed to have two values, one of 7 '8 when it is linked to 

 two different atoms, single linkage as it is termed, and another 

 of 1 2 '2 when doubly linked to one and the sam.e atom of another 

 element. If we subtract six times 7*8, that is deduct the 

 specific volume due to six atoms of oxygen, from the specific 

 volume 1 30 "2 of phosphorous oxide, we arrive at the number 

 83-4 for four atoms of phosphorus, or 20-9 for that of one atom. 

 If any of the oxygen atoms were doubly linked the number would 

 be considerably less than 20 9, hence this number represents 

 the greatest possible value of the phosphorus in phosphorous 

 oxide. 



It would appear, therefore, that the phosphorus in phos- 

 phorous oxide possesses the same specific volume as free 

 phosphorus itself, a result of interest in view of the fact 

 revealed by the determinations of molecular weight that there 

 are four atoms of phosphorus in the molecule of the oxide just 

 as there are in the molecule of free phosphorus itself. 



The liquid oxide, considering that it contains such a highly 

 refractive substance as phosphorus, possesses a remarkably low 

 power of refracting light. Its refractive index at 27°"4 C. is 

 only i'5349 for the red line of lithium, and i"56i4 for the blue 

 hydrogen line G. Not only is the refractive index of phosphorus 

 (2 '0677 for the red hydrogen line C) enormously reduced by its 

 combination with oxygen, but the length of the spectrum is 

 reduced to about one-fifth. 



Action of Light. 

 Phosphorous oxide, in the white wax-like solid form in which 

 it usually condenses after distillation, is remarkably sensitive to 

 light. Ten minutes exposure to bright sunshine suffice to turn 

 it red, and after half an hour it is rendered quite dark red. 

 The red substance which is formed is the red modification of 

 phosphorus, but even after several months' exposure the amount 

 produced has never been found to exceed i per cent, of the 

 weight of the oxide. The beautiful isolated crystals obtained 

 by sublimation in vacuo appear to be unaffected by light, but it 

 is a curious fact that if one of them is melted by the warmth of 

 the hand, and the liquid globule afterw ards suddenly cooled to 

 the wax-like form, the latter becomes red on exposure to day- 

 light. Whether the reddening is due to the conversion of small 

 quantities of admixed yellow phosphorus into red phosphorus, 

 or to the decomposition of the waxy form of the oxide by light, 

 there is not yet sufficient data to determine. 



Action of Heat. 

 It has been seen that the oxide boils without decomposition 

 at I73°*i. It may be heated in a closed tube to considerably 

 over 200° without change. It commences to decompose, how- 

 ever, between 210" and 250°, becoming turbid from the separa- 

 tion of solid decomposition products, one of which is free phos- 

 phorus, which becomes more and more deeply coloured until at 

 300° it is quite red. At about 400° the oxide is totally decom- 

 posed into solid products, consisting of both yellow and red 

 phosphorus and phosphorus pentoxide. Occasionally, when 

 only the lower half of the tube has been immersed in the heatmg 

 bath, the formation of crystals has been observed in the cooler 

 portion of the tube, which appear to be identical with some 

 described in a previous communication to the Chemical Society 

 which gave numbers on analysis agreeing with the formula 

 P2O4, and which yielded a solution with water capable of re- 

 ducing mercuric chloride to calomel. Hence the final decom- 

 position by heat may be expressed by the equation 



5P4O6 = 6P2O5 + 8P, 

 but under suitable circumstances the intermediate formation of 

 the tetroxide may occur according to the equation 

 2P4O6 = 3P2O4 + 2P. 

 Action of Oxygen. 

 Phosphorous oxide takes up oxygen spontaneously at the 

 ordinary temperature. It is provable, however, that the 

 oxygen only reacts in the cold with the vapour, For if a small 

 quantity of the oxide is placed at the bottom of a glass tube 

 closed at one end and previously filled with oxygen, and the 

 tube is sealed and left in the dark in an upright position, the 

 oxide is gradually converted to a voluminous mass of pentoxide. 



NO. II 67, VOL. 45] 



