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tbreeyears from henceforth, and the lease 

 is not executed till the second of January ; 

 in this case the words from henceforth 

 shall be accounted from the delivery of 

 the deed, and not by any computation 

 from the date. And "if the lease be de- 

 livered at four o'clock in the afternoon on 

 the said second day, it shall end the first 

 day of January, in the third year ; the law, 

 in such computations, rejecting all frac- 

 tions or divisions of the day. 



CONCAVE, an appellation used in 

 speaking of the inner surface of hol- 

 low bodies, but more especially of sphe- 

 rical ones. 



CONCAVE glasses, such as are ground 

 hollow, and are usually of a spherical fi- 

 gure, though they may be of any other, as 

 parabolical, &c. All objects seen through 

 concave glasses appear erect and dimin- 

 ished. 



CONCENTRATION, in chemistrv, the 

 act of increasing the strength of fluids, 

 which are rendered stronger by abstract- 

 ing a portion of the mere menstruum. 

 This is generally effected by evaporation, 

 where the menstruum is driven off at a 

 lower heat than is required to drive off 

 the substance with which it is united. 

 Thus, dilute sulphuric acid may be con- 

 sidered as a mixture of the real acid with 

 water ; and by applying a certain heat 

 the water may be evaporized, leaving the 

 acid behind in a state of concentration. 

 AVhen concentrated as much as possible, 

 its specific gravity is about twice as great 

 as that of water ; but it can rarely be ob- 

 tained denser than 1.85. When concen- 

 trated to 2.000 it contains a considerable 

 portion of water, as has been proved by 

 combining it with barytes or potash, in 

 which case water remains behind, and 

 does not enter into the combination. 

 Again, vinegar consists of an acid and wa- 

 ter, and brandy of alcohol and water ; and 

 in proportion as the acid and alcohol are 

 freed from the water, they are said to be 

 more or less concentrated. This may be 

 performed, (1.) either by simple distilla- 

 tion, in which case the acid or spirit comes 

 over first, leaving the water behind ; or, 

 (2,) by exposing the vinegar or brandy to 

 severe frost, when the water will be fro- 

 zen, and the acid or alcohol will be found 

 in a state of concentration in the middle 

 of the ice ; the greater the cold the high- 

 er the state of concentration. M. Lowitz 

 has found that the acid, however concen- 

 trated, congeals at 22. Sulphuric acid, 

 on the other hand, exposed to a much less 

 severe cold, crystallizes, and to effect this 



it must not be greatly concentrated, 

 (3.) Another mode of concentrating the a- 

 cetic acid is by distilling acetate of copper 

 reduced to a powder in a retort. At first 

 there comes over a liquid nearly colour- 

 less, and almost insipid, and afterwards 

 a highly concentrated acid, tinged with 

 green ; but being distilled a second time 

 in a moderate heat, it is colourless, trans- 

 parent, exceedingly pungent, and con- 

 centrated. (4.) The most perfect method 

 of obtaining this acid in a concentrated 

 state was discovered by Mr. Lowitz, of 

 Petersburg!!. It is thus : distil a mixture 

 of three parts of acetate of potash, and 

 four parts of sulphuric acid, till the ace- 

 tic acid has come over into the receiver. 

 To separate it from the sulphuric acid, 

 with which it is slightly contaminated, it 

 must be distilled over a portion of ace- 

 tate of barytes. 



CONCENTRIC, in mathematics, some- 

 thing that has the same common centre 

 with another : it stands in opposition to 

 eccentric. Concentric is chiefly used in 

 speaking of round bodies and figures, or 

 circular and elliptical ones, ,Scc. but may 

 be likewise used for polygons, drawn pa- 

 rallel to each other upon the same centre. 

 The method of Nonius, for graduating 

 instruments, consists in describing with 

 the same quadrant 45 concentric arches, 

 dividing the outermost into 90 equal 

 parts, the next into 89, &c. 



CONCEPTION, in logic, is the simple 

 apprehension or perception which we 

 have of any thing, without proceeding 

 to affirm or .deny any thing about it. 

 There are rules by which we may guide 

 and regulate our conceptions of things, 

 which is the main business in logic : 

 for most of our errors in judgment, and 

 the weakness, fallacy, and mistakes of 

 our argumentation, proceed from the 

 darkness, confusion, defect, or some other 

 irregularity, in our conceptions. The 

 rules are these : 1. To conceive of things 

 clearly and distinctly in their own na- 

 tures. 2. Completely in all their parts. 

 3. Comprehensively in all their proper- 

 ties and relations. * 4. Extensively in all 

 their kinds. 5. Orderly, or in a proper 

 method. 



CONCESSION, in rhetoric, a figure 

 whereby something is freely allowed that 

 yet might bear dispute, to obtain some- 

 thing 1 that one would have granted to 

 him, and which he thinks cannot fairly 

 be denied, as in the following conces- 

 sion of Dido, in Virgil : 



