“If you would be pungent, be brief”

A popular saying about speaking is:
   
“If you would be pungent, be brief—for it is with words as with sunbeams, the more they are condensed the deeper they burn.”
 
The line was published (uncredited) in the Boston (MA) Daily Bee on December 23, 1850, and was reprinted in many other American newspapers.
   
The saying has been credited to English poet Robert Southey (1774-1843) since the late 1800s, but Southey died before the first printed citations—making it improbable that he ever said it.
   
   
23 December 1850, Boston (MA) Daily Bee, “News Items,” pg. 2, col. 2:
If you would be pungent, be brief—for it is with words as with sunbeams, the more they are condensed the deeper they burn.
 
Chronicling America
2 January 1851, Gallipolis (OH) Journal, pg. 1, col. 6:
If you would be pungent, be brief—for it is with words as with sunbeams, the more they are condensed the deeper they burn.
 
6 January 1851, Cleveland (OH) Daily Plain Dealer, pg. 4, col. 1:
[From the Albany Dutchman.]
Crumbs for All Kinds of Chickens.
(...)
If you would be pungent, be brief—for it is with words as with sunbeams the more they are condensed the deeper they burn
 
Google Books
May 1854, Arthur’s Home Magazine, “Gems of Thought,” pg, 394, col. 1:
If you would be pungent, be brief, for it is with work (sic) as with sunbeams, the more they are condensed the deeper they burn.
 
Google Books
A Dictionary of Thoughts:
Being a Cyclopedia of Laconic Quotations
By Tryon Edwards
New York, NY: Cassell Publishing Company
1891
Pg. 52:
If you would he pungent, be brief ; for it is with words as with sunbeams — tin: more they are condensed, the deeper they burn.— Southey.
 
Google Books
The Complete Book of Practical Proverbs & Wacky Wit
By Vernon McLellan
Wheaton, IL: Tyndale House Publishers
1996
Pg. 245:
If you would be pungent, be brief, for it is with words as with sunbeams; the more they are condensed, the deeper they burn. Robert Southey
 
Google Books
10 Things Every Writer Needs to Know
By Jeff Anderson
Portland, ME: Stenhouse Publishers
2011
Pg. 235:
If you would be pungent, be brief, for it is with words as with sunbeams—the more they are condensed, the deeper they burn. —Robert Southey .