SULPHURIC ACID 459 claimed for a calico printer at Rouen, with improvements by Chaptal, such claimants giv- ing credit to Dr. Roebuck only for the inven- tion of the leaden chambers in which the pro- cess was carried on. A large and long cham- ber, divided into sections by partitions which alternately leave open spaces at the top and bottom, has at one end a small furnace in which the flame of sulphur heats a crucible containing a mixture of nitre and oil of vit- riol. The chamber is lined with sheet lead, and its floor is covered with a thin stratum of water. Jets of steam are also introduced. The sulphur in burning produces sulphurous anhy- dride, S0 2 , w r hich in the presence of moisture becomes sulphurous acid (S02 + H 2 0=H 2 S0 3 ), and this again, by the action of N0 6 , be- comes sulphuric acid, H 2 SO 4 , the nitric acid being at the same time reduced to a lower oxide. According to the researches of "Weber and Winkler, the following is the rationale of the process : The oxidation of the sulphurous acid to sulphuric acid takes place in the leaden chambers under the influence of the vapor of water, at the expense of the oxygen of the nitric or nitrous acid, which is converted into deu- toxide of nitrogen. It is necessary however that the nitrous acid be first absorbed in plenty of water, which takes up the free nitrous acid and decomposes the deutoxide of nitrogen, a process greatly promoted by the presence in the chamber of sulphurous acid purposely in- troduced. The water, usually in the form of steam (practical experience proving that a cer- tain elevation of temperature is required), acts in this process as in others wherein sulphurous acid effects reduction. By the presence of at- mospheric air in the chamber the deutoxide of nitrogen is oxidized into hyponitric or ni- trous acid, and this acid again is decomposed by sulphurous acid. A peculiar crystalline sub- stance sometimes forms, having the formula H 3 S0 4 + N" 2 3 ,S03, and formerly thought to play FIG. 2. Class-room Apparatus for Sulphuric Acid. an important part in the transformation ; but according to R. Weber this substance only ap- pears when the process is not well managed, and is chiefly due to want of water. The pro- cess of forming sulphuric acid may be illustra- ted on a small scale by means of the apparatus shown in fig. 2. A flask, 5, furnishes sulphu- rous anhydride, and the bottle e deutoxide of nitrogen, to the large glass balloon r, and the flask w supplies steam when it is required. Air is occasionally blown into the balloon through the bent tube t, the effete products passing out at o. If but little vapor of water is present, the white crystalline solid above mentioned makes its appearance upon the sides of the globe ; but when sufficient water is present the substance is not deposited, neither is it sup- posed to be formed as a necessary stage of the process. Gay-Lussac invented what is called a condenser as an attachment to the large leaden chambers, for the purpose of- economizing the consumption of nitre, which formerly amount- ed to from one eighth to one twelfth of the weight of sulphur. The condenser consists of a leaden tower filled with fragments of coke, through which sulphuric acid of 66 Baum6 is constantly trickling. Through this con- denser the spent gases are passed, and the oxides of nitrogen which they may contain are absorbed. The sulphuric acid which col- lects at the bottom of the chambers is too dilute for most purposes ; it is not found ad- vantageous to allow it to attain a specific grav- ity of quite T6, because at that strength it absorbs too much of the nitrous fumes. It may be used at this strength for the manufac- ture of salt cake (see SODA), but for other uses it must be further concentrated. This is generally effected by two different stages, the first in leaden pans, the second in platinum or glass retorts. Some makers concentrate to 60 Baume (sp. gr. l'7l) in leaden pans; others use them only till the acid is raised to 55 Baume" (sp. gr. 1-59). The leaden pans are rectangular, long and wide, supported by iron plates to protect the lead from imme- diate contact with the flame which is used for evaporation. The pans are generally arranged in steps, the acid being conveyed from the up- per to the lower ones by syphons, the den- sity of the acid increasing from one pan to the next lower. "When it has attained a den- sity of from 1-65 to 1/72 in the leaden pans, it is known as brown oil of vitriol, and may be used by bleachers, calico printers, dyers, &c. ; but to raise it to the strength of com- mercial oil of vitriol, it is further concen- trated in the glass or platinum retorts above mentioned. Glass retorts holding 20 gallons or more are often used, set in an iron pot, the bottom of which is covered with dry sand. The concentration requires from 12 to 16 hours; the vapors which distil over toward the last, carrying some acid with them, are passed into condensers and returned to the lead pans. Platinum retorts are more costly, but are thought by many to be more economical in the end on account of their not being liable to break. Fig. 3 shows a section of a platinum retort. The syphon x is worked without a stopcock by the vessel c, in the following in- genious manner. When the vessel is lowered
