If food is allowed to stand for some time, it putrefies .When the putrefied material
3 W- [ H5 k% N P6 dis examined microscopically ,it is found to be teeming with bacteria. Where do these 6 E1 ?0 U/ Y+ B1 X
bacteria come from , since they are not seen in fresh food? Even until the mid-nineteenth 6 p5 u+ S" X" Z' `9 }
century, many people believed that such microorganisms originated by spontaneous( V0 \: o X+ a8 Q1 U( [
(5 ) generation ,a hypothetical process by which living organisms develop from nonliving
2 Q7 o7 B* j+ A# H+ |5 Zmatter.
. U0 H( z. ]3 f7 WThe most powerful opponent of the theory of spontaneous generation was the French chemist and microbiologist Louis Pasteur(1822-1895).Pasteur showed that structures
" B6 u6 a# h& B1 Z% {- n( }. }present in air closely resemble the microorganisms seen in putrefying materials .He did ) ^% S( `( {# [" v1 M- p
(10) this by passing air through guncotton filters, the fibers of which stop solid particles. After
9 |7 P1 z: k. t! U& V: pthe guncotton was dissolved in a mixture of alcohol and ether, the particles that it had
% D' S3 L* x" Ltrapped fell to the bottom of the liquid and were examined on a microscope slide .Pasteur
, n( `- Y) H4 [: L; i' _7 o& ]) Ffound that in ordinary air these exists a variety of solid structures ranging in size from $ Q, j5 k+ F/ W1 ^+ ?% n! r+ f! }, n) E
0.01 mm to more than 1.0 mm .Many of these bodies resembled the reproductive
( a3 E1 a5 j8 j( W) J(15)structures of common molds, single-celled animals, and various other microbial cells .5 M: ?6 l3 y" a0 s( o/ a
As many as 20 to 30 of them were found in fifteen liters of ordinary air ,and they could+ y0 {7 l; N) Q$ V, j+ s' P1 w5 D
not be distinguished from the organisms found in much larger numbers in putrefying materials .Pasteur concluded that the organisms found in putrefying materials originated6 d2 { ~( G) l! q9 H
from the organized bodies present in the air .He postulated that these bodies are constantly2 ^; k% y: W: u Y
(20)being deposited on all objects.2 c2 z- Q5 O* H6 j
Pasteur showed that if a nutrient solution was sealed in a glass flask and heated to
! E3 l W3 k( _; rboiling to destroy all the living organisms contaminating it, it never putrefied .The proponents of spontaneous generation declared that fresh air was necessary for $ B Y+ n3 x8 j+ V& j
spontaneous generation and that the air inside the sealed flask was affected in some way
$ B% W& b3 m2 q; k5 \0 @(25)by heating so that it would no longer support spontaneous generation. Pasteur constructed a swan-necked flask in which putrefying materials could he heated to boiling, but air
9 Y6 o# J+ Q2 u1 r+ r: bcould reenter. The bends in the neck prevented microorganisms from getting in the flask.. Material sterilized in such a flask did not putrefy.) n8 w) v. o! G# |
, m4 X; R& d9 j9 O% L8 h, ^ `1,What does the passage mainly discuss?
( B. D6 X8 F* g4 q V(a)Pasteur’s influence on the development of the microscope.
5 M$ }7 u+ t0 A: t; b, C(b)The origin of the theory of spontaneous generation .
2 o8 |2 \1 B" ], q1 c/ J; b(c)The effects of pasteurization on food.3 e! @ v6 o! }/ a" K- d
(d)Pasteur’s argument against the theory of spontaneous generation .
+ ?) A7 n* |! y2,The phrase “teeming with ”in line 2 is closest in meaning to
( T! H% N$ y0 m3 ~(a)full of) c. N) e6 w0 m( o
(b)developing into8 c* b1 _% t0 G- n# F3 y; G
(c)resistant to
- b) A% s8 A4 p(d)hurt by |
发表于 2012-8-14 22:34:35
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