Good afternoon,
First thing you need to know is whether the author is deceased. If yes when did he die, copyright lasts for 70 years after death for written work.
There are then limits on how much you can copy if still in copyright. It was 10% but OH (senior library assistant) says it changed recently. She says 20% for personal use, ie, not being republished or used in anyway for public consumption. As you want to use it online that is not personal use so a smaller amount only allowed.
John915
Sorry but there have never been percentage limits on copyright.
What you have written above is simply guidelines for libraries and not legally enforceable.
Copyright law cases have shown that even as few as 15 words can be a breach of copyright.
What counts is not the percentage of the work used but whether you have used the "essence" of the work such that it diminishes or destroys the market for the original.
In the specific case published in 1878 the author is dead, this basically means if he/she died prior to 1946 the copyright has expired (UK law other countries allow for extensions).
In the circumstance supplied there is little doubt the he/she would have died prior to 1946.
A secondary concern could also be if there were subsequent publications of the book in the intervening years as if there was only one publication and the book had not been available new for some time any court would be less likely to uphold a claim of copyright infringement as there would be no diminishing or destruction of the market.
Cheers
Guy