629 



DIFLUAN. 



DIGESTER. 



E30 



will be found some numerical results obtained by comparing togethe 

 the squares of the times in which equal quantities of different salts ar 

 diffused. The second column shows the relative difi'usibility of th 

 different groups of the hydrochloric acid group ; the third colum 

 gives the times required for the diffusion of equal weights of th 

 members of each group ; and the fourth shows the ratio of the square 

 of those times of equal diffusion. 



Ratio of squares 



Rale of Times of equal of times of 



Groups. diffusion. diffusion. equal diffusion. 



1. Hydrochloric acid . 1-000 3-960 2 



2. Hydrate of potash . 0-800 4-950 3 

 S. Nitrate of potash . 0-565 7-000 6 



4. Nitrate of soda . . 0-462 8-573 9 



5. Sulphate of potash . 0-400 9-900 12 



6. Sulphate of soda . . 0-326 12-125 18 



7. Sulphate of magnesia 0-200 19-800 48 



III. Liquid diffusion is most regular in dilute solutions. It wa, 

 also found that the quantity of a substance diffused from a solution o 

 uniform strength increased with the temperature, but that the ratio o 

 diffusion between different bodies at the same temperature was con 

 stant. It was found also, that of the whole quantity rather more than 

 one-fourth was diffused during the first two days; the quantities 

 diffused during each remaining period of two days being very nearly 

 equal. IV. When two substances which do not combine chemically 

 but having different degrees of diffusibility, were mixed in solution 

 and placed in a diffusion cell, the more diffusible substance passed oul 

 more rapidly than the other, so that a partial separation of the two 

 bodies could be effected by this means. Thus the quantities of the 

 carbonates of soda and of potash, diffused in the same time from a 

 solution containing equal parts by weight, were as the numbers 35 and 

 65 nearly. It was even found, that in some cases chemical decomposi- 

 tion could be produced by liquid diffusion. Thus a solution of com- 

 mon alum (which is a compound of sulphate of alumina and sulphate 

 of potash) arranged so as to diffuse into water, the sulphate of potash 

 passed out more rapidly than the sulphate of alumina. V. It was 

 found in the case of dilute solutions that one substance will diffuse 

 into water which already contained another substance in solution, just 

 as into pure water. 



The reader who desires to pursue this subject further, is referred to 

 Professor Graham's Papers, contained in the ' Philosophical Transac- 

 tions ' for 1850. 

 DIFLUAN. [URIC ACID.] 

 DIGAMMA, or VAU, is the name given by grammarians to a letter 

 which once belonged to the ancient alphabet of the Greeks. It 

 appears to have occupied the sixth place in that alphabet, for while 

 eptilon is employed as the numerical symbol forjire, the next letter, 

 an that alphabet is now arranged, is the representative of seven. 

 Moreover, this position of the digamma will correspond precisely with 

 that of rait or wa/ of the Hebrew, and of / in the Latin alphabet, two 

 letters of kindred power and form. A further argument may be 

 found in the principles which would seem to have determined the 

 arrangement of the Greek alphabet. [ALPHABET, vol. i., col. 235.] The 

 letter i gtill to be seen in many inscriptions. [ALPHABET, vol. i., 

 cols. 240-2, plate ii., Nos. 20, 21, 26.] With regard to the power of 

 the letter, it is now the general and well-established opinion that it is 

 equivalent to our own w. Its name has been evidently derived from 

 the similarity of its symbol F to a repetition of the Greek t/amma F. 



The use of the digamma prevailed more particularly in the ^Eolic 

 dialect of the Greek tongue. In the other dialects it was commonly 

 dropped, particularly the Attic ; and as this became the favourite 

 dialect of Grecian literature, the digamma at last escaped from the 

 alphabet ; and even the Homeric poems, which had been written in a 

 dialect still possessing the digamma, were presented to, the Athenians 

 without that letter, to the serious injury of the metre. But though 

 the form of the digamma was not admitted into the Attic alphabet, 

 the vowel o was occasionally used, so as virtually to represent it, as in 

 0180, OIKOJ, omot, equivalent to FIAA, FIKO2, FINO2 (comp. the Latin 

 video, now, rimun) ; and it was altogether superfluous to prefix the 

 digamma, FOIKO2, as was sometimes done. Other substitutions for the 

 digamma are /3, as ftoivoy for fotvos or otvos, (&cua for (fata or (out, fipo&ov 

 for Fpooov or pooov, ptC a f r ^P'C a or ^'C a (Cf- Germ, wurz-el and our wort = 

 root) ; secondly, ou, as Ouappiav for Varro ; thirdly, u, as oupTjirrot for 

 u-Fpittrot or appifKTot, auat for aFus or TJMJ. Whether y is ever sub- 

 stituted is disputed. Some, supported by the analogy of the French 

 habit, as gdttr for vattare, and the repeated authority not only of the 

 Hesychian glosses but of MSS., hold that ytttv, yei/tara or yntwra, 

 ytrro are genuine dialectic varieties of F(8*r, Fft^ara, FfvTo. Ahrens 

 on the other band regards the T as a clerical error for F. A still more 

 violent substitution is that of p for F, as SeSpoiKias for StSfoixcos or 

 8Jois, of which we have the converse in our mtbbish for rubbish. 



The Latin language, being more closely connected with the ^Eolic 

 dialect of the Greek, U abundant in the use of this letter ; for the true 

 pronunciation of the v or u contonnns must have been the same as 

 our w, or it could not have so readily interchanged with the vowel . 

 The Greek words aav, tap, emr.pos, ecr-no, tor, appear in Latin as mum, 

 ver, retperia, reita, riota. In the last instance indeed there are two 

 other point* of difference, the Latin word being feminiiie, like rosa 

 ARTS AKO SCI. I)IV. VOL. III. 



compared with the Greek f>oSoi> ; and secondly, a diminutive, which is 

 well suited to the size of the flower. Sometimes a b appears in the 

 Latin word, where the ^Eolic Greek must have had the digamma, as 

 probta compared with wpaiis ; or an /, Formiie compared with Ormice. 



The disappearance of the digamma in one dialect and its retention 

 in another is in perfect accordance with what is seen in modern 

 languages. In our own we have ceased to pronounce the w in who, 

 whose, tu-o. sicord, answer, whole; while in one and once we have the 

 sound without the character, and yet drop it again in only. The Danish 

 dialect of the Teutonic language is remarkable for throwing off the -, 

 thus word in the mouth of a Dane is ord. 



For the assertions of the grammarians and the opinions of the 

 learned with regard to the digamma, see Kidd's edition of Dawes's 

 ' Miscellanea Critica,' pp. 175-335. The editor has given a list of the 

 Greek words which he supposes once to have possessed this letter. A 

 more complete discussion of the subject is to be found in Ahrens, 'De 

 Gracfe Linguae Dialectis.' 



DIGEST. [CORPUS JURIS ; JUSTINIAN'S LEGISLATION.] 



DIGESTER. A strong steam-tight vessel, usually of iron or copper 

 in which water and other materials can be heated considerably beyond 

 their boiling points; this form of apparatus was first contrived by 

 Papin, and is hence frequently termed Papin's digester. The following 

 is a description of two vessels of this description which are frequently 

 employed by chemists. 



The first is constructed entirely of Low Moor wrought-iron, and con- 

 sists of a cylinder A A (firj. 1), closed at the bottom, and welded in one 

 piece by the steam-hammer. The 

 cylinder is 18^ inches long, 5 inch Fi ff- * 



in thickness, and 3 inches internal 

 diameter; it is furnished at top 

 with a flanch, B B, 1| inch broad 

 and f inch thick, its upper surface 

 ;urned true, and having an internal 

 annulus sunk inch below the level 

 of the surrounding surface. The 

 cap of the digester, c c, is made to 

 it upon this flanch, with which it 

 corresponds in thickness and dia- 

 meter ; it is furnished with a pro- 

 ecting face J inch deep, fitting the 

 nouth of the cylinder exactly. 

 Within the circle of this project- 

 ng face, the cap is perforated by 

 .wo apertures, into one of which 

 s securely fixed the cast-iron tube 

 I d, closed at the bottom, 6 inches 

 ong and J inch internal diameter, 

 orming a mercury bath for the 

 eception of a thermometer. The 

 ither aperture, which is bouched 

 vith brass, serves as the bed of the 

 safety valve e , which consists of a 



iece of brass wire J inch diameter, slightly flattened on two sides and 

 urnished with a head accurately ground to the surface of the cap : 

 treasure is applied to this valve in the usual manner by the lever and 

 weight / </. Both the flanch and cap are perforated by four holes for 

 he reception of four screw bolts 4 inch in diameter, which are inserted 

 rom below, and work into nuts that can be tightly screwed up by a 

 ever key. The whole of the pressure produced by these screw bolts 

 s made to take effect exclusively upon the surface of a leaden washer 

 inch thick, placed in the sunken annulus above mentioned : and thus 



Fig. 2. 



lie apparatus is made perfectly impervious to gases 

 nd vapours, even under the enormous pressure of 

 more than 100 atmospheres. In this digester, vola- 

 ile liquids enclosed in glass tubes of large dinieu- 

 ious and moderate thickness of glass, may be 

 xposed to any temperature below redness with 

 vfety. Water is generally used in this digester, 

 ut other and less volatile liquids may of course 

 substituted if desired : in most experiments, 

 owever, it is important that the pressure upon 

 ie exterior of the glass tubes should not be much 

 ess than that in their interior, and this condition 

 s generally secured by the employment of water in 

 le apparatus. 



The second digester is made of wrought copper and 

 9 of smaller dimensions, being especially designed 

 >r the heating of chemical substances without the 

 ntervention of glass tubes. It consists of a wrought 

 >pper tube, A A (jty. 2), 18 inches long, 1J inch 

 iternal diameter, and \ inch in thickness, drawn 

 rom a solid mass of the metal by a recently invented 

 rocess. This tube is closed at bottom by a screw 

 lug, and is furnished at top with a brass flauch, B B, 

 | inch broad and 1 inch thick, screwed upon the 

 opper tube : the vessel thus formed is closed by the 

 rass cap c c, of the same dimensions as the flanch upon which 

 fits. The cap is furnished with a central projection 1 inch deep, 



