122 



POPTJLAE SCIEITCE JSTEWS. 



[August, 1888. 



nection between the physical and chemical 

 properties of different substances. In the 

 department of organic chemistry, or the chem- 

 istry of the hydrocarbon compounds, the work 

 to be accomplished which is alread}- in view 

 is almost endless. In these numerous sub- 

 stances made up of carbon, hjdrogen, and 

 oxygen, we have the most diverse properties 

 depending apparentl}' upon the smallest pos- 

 sible variation in composition, or even in 

 those made up of the same kind and number 

 of constituent atoms. In organic substances 

 the actual composition seems to count for 

 very little, while the wide difference between 

 different hydrocarbons apparently depends 

 upon some other cause, which, for want of a 

 better explanation, we call the relative position 

 of the atoms of which the molecule is built 

 up. While this theory perfectl}' explains the 

 obsei-ved facts, it is, on the whole, a not very 

 satisfactory one ; and to discover the actual 

 property or attribute of the substances 

 carbon, liydrogen, and oxygen which causes 

 them to combine into the innumerable and 

 widely differing bodies alread3' known to us, 

 is a problem of equal importance with that of 

 the nature of the elements themselves. 



.Turning to the practical side of the ques- 

 tion, the opportunities for the application of 

 chemistry to the advancement of the arts are 

 very numerous. The artificial manufacture 

 of natural products will doubtless be greatly- 

 extended. Already the brilliant coloring- 

 matter of the madder-plant isimade artiflciallj' 

 in immense quantities, and the magnificent 

 blue of the precious lapis-lazuli is exactlj' 

 copied in the common ultramarine ; coal-tar 

 furnislios a substitute for sugar in the weU- 

 known saccharin ; while indigo, glucose, cer- 

 tain alkaloids of the quinine group, and many 

 other valuable substances, have been directly 

 formed from their elements, although as yet 

 at a cost which prevents the practical ap- 

 plication of the methods. It is not too much 

 to hope that processes of manufacture mav be 

 discovered by which such substances as mor- 

 phine, quinine, indigo, etc., can be produced 

 at a cost much below the present. 



The utilization of waste products is an 

 economical problem not to be overlooked, and 

 the destruction or purification of sewage is 

 also of the highest importance. Substances 

 possessing new and unusual properties are 

 constantly needed in the arts, and the chemist 

 will find a remunerative employment in supply- 

 ing this need. Perhaps the most important 

 discovery of all, however, would be a means 

 for converting the energy stored up in coal, 

 directly into electricity without waste. If this 

 is ever done, it will produce a greater indus- 

 trial revolution than the introduction of the 

 steam-engine. Light, heat, and power could 

 be produced at a much smaller cost than at 

 present, and distributed in any direction. 

 Although we can see no wa}- at present by 

 which this result can Vje attained, it is not 

 impossible, and may become a fact before 

 another hundred years has passed away. 



The progress of chemistry has been very 



great ; but it is not a progress towards a jour- 

 nej^'s end, but into an ever-widening field for 

 discovery which gives a jiromise of an even 

 more brilliant future. 



PRECIPITATION. 



If we dissolve some ordinary washing or 

 cooking soda in water, and add it to lime- 

 water, the mixture will become turbid, and a 

 white opaque powder will be formed, which 

 will finally subside to the bottom of the vessel. 

 This phenomenon is known as precipitation, 

 and always occurs when a substance in solu- 

 tion is from any cause rendered insoluble. 



In the experiment above referred to, two 

 substances, h3drate of lime and carbonate of 

 soda, both of which are soluble in water, were 

 mixed together, when a chemical change took 

 place by which the acids and bases changed 

 places with each other, forming hydrate of 

 soda and carbonate of lime. Carbonate of 

 lime, however, is quite insoluble in water, and 

 therefore separates or is precipitated out of 

 the solution. 



This process of precipitation is of the utmost 

 importance to the chemist, and is the chief 

 means by which, in the analysis of unknown 

 substances, he separates out the different con- 

 stituents. Nearl}^ every element can be pre- 

 cipitated by some reagent ; and the appearance 

 of the different precipitates is in many cases 

 so characteristic, that the nature of the sub- 

 stance under examination is at once evident. 



Suppose, for instance, that we have a clear 

 liquid containing salts of some unknown met- 

 als in solution. We first add a quantity of 

 hydrochloric acid, and a copious white precip- 

 itate is at once formed. This may indicate 

 the presence of either lead, mercury, or silver ; 

 and it is necessary to still further separate the 

 constituents. We filter and wash the precip- 

 itate, and then boil a part of it with water. 

 The hot water dissolves the chloride of lead, 

 which cr3-stallizes out on cooling ; or we may 

 add a little dilute sulphuric acid, which again 

 precipitates the lead in the form of a sulphate. 



Upon the remainder of the precipitate upon 

 the filter we pour strong ammonia. If it turns 

 black, mercury is present. If it also contains 

 chloride of silver, this will dissolve in the 

 ammonia, and pass through the filter ; and by 

 carefully neutralizing the ammonia with dilute 

 nitric acid, the chloride of silver is again pre- 

 cipitated by itself, and can be collected and 

 weighed if necessary. In the same manner 

 all the different elements can be separated 

 from each other by appropriate reagents, 

 their presence detected, and their amount 

 determined. 



The appearance of precipitates varies very 

 greatly. The carbonate of lime mentioned 

 above is a light white powder. Sulphates of 

 lead or barium are very heavy and dense 

 white powders. Hydrate of iron precipitates 

 as a slimy brown mass, which, upon boiling, 

 becomes of a more granular nature. Hydrate 

 of alumina is flocculent, like cotton-wool ; while 

 the ammonio-phosphate of magnesia is precip- 



itated in minute crj-stals, and more abundantly 

 in the places where the sides of the vessel con- 

 taining it have been rubbed with a stirring rod. 

 Phosphate of manganese is a gelatinous mass, 

 but, upon boiling, is changed into miiuile scales 

 of a pearly lustre, which can be filtered and 

 washed with the greatest ease. Gold, silver, 

 and platinum can be precipitated in the meta 

 lie state as black powders of inconceivabli 

 fineness, or wlien separated by a current ol 

 electricity, as in electroplating, show their 

 usual metallic cnaracieristics. Chloride of 

 silver resemhles curds of milk, and, in addi- 

 tion, turns black in the sunlight. 



The colors of precipitates are verj' varied 

 and characteristic. Sulphide of iron and most 

 of the metallic sulphides are black ; sulphide 

 of arsenic, however, is of a bright yellow, and 

 sulphide of antimony a vivid orange ; hj'drate 

 of chromium is green, and ferrocyanide of 

 iron a magnificent blue (Prussian blue). A 

 compound of gold and tin is the beautiful pur- 

 ple of Cassius. Iodide of mercuiy, when first 

 precipitated, is of a yellow color, but in a few 

 minutes changes spontaneously to a brilliant 

 scarlet ; a change of cr3-stalline form occurring 

 at the same time. 



Precipitation may also occur from a change 

 in the nature of the solvent as well as that of 

 the substance in solution. We have seen how 

 the chloride of silver is precipitated from its 

 solution in ammonia when the latter is neu- 

 tralized with an acid. By diluting a solution 

 of resin in alcohol with sufficient water, the 

 alcohol may be so weakened that the resin is 

 precipitated, and the liquid becomes cloudy 

 and opalescent. A few drops of cologne, 

 which is an alcoholic solution of various per- 

 fumes, poured into a glass of water, will illus- 

 trate this very clearly. Chlorides of antimony 

 and bismuth in strong acid solutions are also 

 decomposed by dilution with water, basic salts 

 being precipitated. 



Potassium and sodium are precipitated with 

 difficulty, as nearl}' all their compounds are 

 soluble. A chloride of platinum and potassium 

 is, however, insoluble in alcohol ; and a com- 

 plex salt of antimony, tartaric acid, and so- 

 dium is not very soluble. Sodium, however, is 

 rarely precipitated in the course of an analysis, 

 but is usually separated and determined by 

 other methods. 



To the chemist nothing can be more inter- 

 esting than the appearance of the different 

 precipitates as the result of his mixing of dif- 

 ferent solutions. The appearance of a slight 

 turbidity in a clear solution may be an indica- 

 tion of a discovery of the utmost importance. 

 We are still unable, however, to answer the 

 question- as to why precipitation takes place. 

 We do not know why hydrate of Mme is solu- 

 ble in water, and carbonate of lime insoluble ; 

 and that quality of matter which renders it 

 soluble or insoluble must be left for future 

 experimenters to discover. 



Paper Houses. — At Parkersburg, Penn., two 

 houses are being erected, which will have paper 

 walls, paper partitions, and paper roofs. 



