2s8 



NA TURE 



[July 12, 1S94 



rough towel, which is responsible for a considerable portion of 

 the refrtS'hing influence of the British bath. 



Besirfes these sudoriferous glands, however, there is a second 

 set, called the sebaceous glands, the ducts of which are spiral, 

 and open generally into little pits, out of which the fine hairs 

 which stud the skin grow, and these glands secrete an oily or 

 waxy substance, which nourishes the hair, and also keeps the 

 outer skin smooth and pliant. This waxy substance is deve- 

 loped in largest quantity inside the ear, where it serves to pro- 

 tect the more delicate portions of that organ, and next to the 

 ear, these glands are found most abundantly on the face and 

 other portions of the body which are exposed to external 

 influences and friction. 



It is the presence of this oily secretion which holds the dirt 

 glued to the skin, and being also rubbed off on the inside of the 

 wristbands and collars of our shirts, causes these portions of our 

 linen to become the most soiled. We may look upon thi-; form 

 of dirt, therefore, as being glued on to the surface by oleaginous 

 materials, which being insoluble in water resist any mere rinsing, 

 and the most important function of our cleansing materials is to 

 provide a solvent which shall be able to loosen the oil, and so 

 allow of the removal of dirt from the skin. 



The skin, however, is not the only source of oily matter, and 

 in all fibres of animal origin more or less fat is to be lound, 

 which although not in sufficient quantity to play any very im- 

 portant part in the fixation of dirt, still adds its iota to the 

 general result. 



We notice, moreover, that the air of a big town has a far 

 greater dirtying effect than country air, this being partly due to 

 the f.TCt that the number of solid particles per cubic foot of atmo- 

 sphere are greatly reduced, but chiefly because country air does 

 not contain ceitain products of incomplete combustion, which 

 are to be found in all large towns. 



In London we annually consume some six million tons of 

 bituminous coals, and if we examine the smoke which escapes 

 up our chimneyduring the imperfect combustion which the coals 

 undergo in our fire-grates, we find ihat not only will that smoke 

 contain small particles of unconsumed carbon in the form of 

 blacks or soot, but also a considerable quantity of the vapour of 

 connensible hydrocarbon oils, which depositing on the surface 

 of the solid particles of floating dirt, gives them an enhanced 

 power of clinging to any surface with which they come in 

 contac. 



If we have a heavy fall of snow in London, as the snow melts 

 it leaves a black deposit, which is formed of the solid particles 

 with which the snow has come in contact in its passage through 

 the air, and a recent analysis of a deposit of ihis character, 

 collected on the glass roof of an orchid house at Chelsea, 

 gives a very good idea of the constituents of these solid im- 

 impurities. 



Hydrocarbon oils of this character are not as a rule affected 

 by the solvents which we utilise for loosening the dirt which is 

 held to our skin by animal grease; but ihtre is no doubt that 

 the dirtying influence of town air is greatly increased by their 

 presence. 



If we take any grease of vegetable or animal origin, we find 

 that it can be dissolved in liquids containing free alkalies, this 

 term being applied to the compounds formed by water with the 

 soluble metallic oxides, which, when dissolved in water, give 

 solutions having a soap-like taste, affecting the colour of vege- 

 table extracts, such as that obtained by the red cabbage, and 

 pojsesiing the power of neutralising the acidulous properties of 

 the c-iuipounds we call acids. 



If we lake two metals discovered by Sir Humphrey Davy in 

 ■ 807, potassium and sodium, anil expose them to dry pure air, 

 they rapidly become convened into a while powder by absorb- 

 ing oxygen from ihe atmosphere, and form compounds which 

 we term retpeclivcly oxide of sodium and oxide of po:assium. 

 These oxides, when dissolved in water, enter into combination 



NO. 1289, VOL. 50] 



»-ith a portion of it, producing sodic hydrate and potassic 

 hydrate, two substances which have pre-eminently the properties 

 which we term alkaline, and which exert a strong solvent action 

 upon all forms of animal and vegetable grease. 



These solutions exercise a wonderful power of cleansing upon 

 the grease-bound particles of dirt which veil our skin, but so 

 strong is their solvent power upon animal membrane, that not 

 only do they dissolve fatly matter, but also the cuticle itself, so 

 that they are manifestly unfitted for removing dirt from a tender 

 skin, and we are forced to look further afield for a grease 

 solvent. 



If instead of dissolving our sodic and potassic oxide in water, 

 we had left them exposed to ordinary air, we should have found 

 that they gr.idually attracted from the atmosphere a gas called 

 carbon dioxide, which exists in all air to the extent of 4 parts 

 in 10,000, and that by combining with this gas they became 

 converted into sodic and potassic c.irbonates, bodies which we 

 call salts, and which, although not so violent in their action 

 upon the skin, will retain to a certain extent their solvent action 

 on fatty matters. 



The carbonates of sodium and potassium are found in the 

 ashes of many vegetable and animal substances, and in the 

 earliest records which have been discovered, we find mention 

 of the cleansing power of wood ashes, the ashes of certain 

 marine plants, sea-weed, and " natron," which is an alkaline 

 efflorescence from some kinds of soil ; nor has the use of ashes 

 for this purpose entirely died out at the present time. 



It was only in 1SS4, that during some structural alterations in 

 Rome, an old tomb was broken into, and the ashes which it 

 contained removed by one of the workmen, who conveyed them 

 home to his wife, as an offering towards the next washing-day, 

 whilst a few days later the antiquarians were horrified to dis- 

 cover that they were the lemains of the Emperor Galba, 

 cremated some eighteen centuries before, which had been put to 

 such practical use. 



As early as ad. 69, however, we find that the elder Pliny 

 mentions another form of cleansing material made from tallow 

 and ashes, the components most recommended being goai\ 

 suet and the ash of beechwood ; whilst the ruins of Pompeii 

 were founrl to contain a fairly perfect soap factory. 



Although soap and Christianity date from the same period, it 

 was only at the commencement of this century that the cl.assic:il 

 researches of Chevrcul on the constitution of fats, gave the kt) 

 to the reactions taking place during its formation, whilst even 

 at the present time we probably only know a true explanation 

 of part of the actions which lead to its cleansing efiect ujn n 

 the skin. 



If we lake sulphuric acid diluted with water, we find that it 

 has certain well-maiketl characteristics, which leave no room 

 for doubting its acidulous nature, and if we pour a lew drops of 

 it into the violet coloured solution obtained by boiling slictl 

 red cabbage in water, the violet solution at once becomes bright 

 red. On repeating this experiment with the violet cabb.ige 

 solution, and a few drops of sodic hydrate solution, 

 we obtain a vivid green colour, and now on mixing ilu- 

 solution rendered red by the .acid, and the second one luriml 

 green by the alkaline base, we once more obtain the original 

 violet colour, an<l on examining the solution can find no tract- 

 of either acid or alkali, hut can distinguish the presence of 1 

 compound called sodic sul|ihate, which can be obtained in il 

 crystalline form by concentrating the solution, and such a C(iii 

 pound furnicd by the union of an acid and a base, we are in ihc 

 habit of calling a salt. During the combination of the sulphuric 

 acid and sodic hydrate to form sodic sulphate, we also had wait-- 

 being formed, which, like the neutral sail, had no action U| 1 

 our coloured solution. If we had carefully weighed our su 

 phuric acid and the sodic hydrate, we should have lound that u 

 is only in certain definite proportions that they unite to give a 

 solution without cflTcct on the vegetable colouring matlei, and 

 we might sum up our experiments on the combination of these 

 two substances .as follows : — 



Acid. 

 Sulphuric acid. , 

 98 pans by wciKiit. 



Rase. 



Sodic hydr.itc, 



60 part!) by weitcbl 



A salt. Water. 



Sodic mlphale, , 3* P»"» 

 141 pans by weight, '''by weight. 



And if we lake crystals of sodic sulphate, am! dissolve them in 

 water, we can decompose ihem once more into sulphuric acid | 

 and sodic hjdralc by the aid of galvanic electricity. 



My aim in this experiment has been to impress upon you that 

 a salt is a compound formeil by Ihc uniim of an acid and a base, 



I 



