First of all, remember that the way people used words 150 years ago does not dictate how we use words today, even though their usage has probably influenced our usage very deeply.
Since especially and specially look and sound so much alike, it can be very difficult to realize the clear differences in their definitions. The fact that special and especial, their adjectival forms, share even closer definitions makes the confusion that much more difficult to sort out.
From www.learnersdictionary.com
The meanings and usage of these two similar-sounding words overlap
quite a bit, so it can be hard to figure out which one to use when. If
you are interested in the details, I encourage you to read their
entries in Merriam-Webster's Learner’s Dictionary. If that’s more
information than you need, here are simple rules to follow that will
insure that you are using these words correctly:
Use especially to mean “very” or “extremely,” as in these examples:
There is nothing especially radical about that idea.
The food was not especially good.
Use especially when something stands out from all the others, and you want the meaning of “particularly,” as in these examples:
She can't be sure she will win, especially at this early stage of
the campaign.
The appetizers and especially the soup were
delicious.
When you want to convey the meaning “for a special purpose,” or “specifically,” you can use either especially or specially. They are
both correct.
The speech was written especially/specially for the
occasion.
When you want to convey the meaning “in a special manner”, as in this example below, use specially. In this context, especially would
sound odd or wrong to most native speakers.
I don't want to be treated specially.
[AVOID] I don't want to be treated especially.
Rule 4 is the most important to remember, because it is the only one where especially will not fit. In all other uses, you can safely assume especially will work.
The more extensive difference between the adverbial forms seems to be related to the subtle difference between their adjectival forms:
The meanings of the adjectives special and especial are more or
less the same. They both mean “different and more than usual,” as in
the examples below:
This is a matter of special importance.
Pay especial attention to the last paragraph.
However, special is by far more common than especial. In fact,
according to COCA, the Corpus of Contemporary American English,
special is used about 600 times for every one time that especial is used.
In addition, unlike especial, special does not need to be followed
by a noun, as in this example:
If you’re going to a business lunch, wear something special.
Since special is much more common, and its use is more flexible, I
don’t recommend using especial at all.
Since especial is fading out of use, you can reduce the confusion by assuming special is the right adjective, unless you have a clear compelling reason to use especial. Interestingly, the 150:1 usage ratio of special over especial is reversed in the 85:1 usage ratio of especially over specially. Especially is significantly more common than specially.
I use the following mnemonic to remember that the adjective special is more useful and the adverb especially is more popular:
Special words are especially important.
